arm64: dts: imx8ulp: correct the flexspi compatible string

In the Linux kernel, the following vulnerability has been resolved:

arm64: dts: imx8ulp: correct the flexspi compatible string

The flexspi on imx8ulp only has 16 LUTs, and imx8mm flexspi has
32 LUTs, so correct the compatible string here, otherwise will
meet below error:

[ 1.119072] ------------[ cut here ]------------
[ 1.123926] WARNING: CPU: 0 PID: 1 at drivers/spi/spi-nxp-fspi.c:855 nxp_fspi_exec_op+0xb04/0xb64
[ 1.133239] Modules linked in:
[ 1.136448] CPU: 0 UID: 0 PID: 1 Comm: swapper/0 Not tainted 6.11.0-rc6-next-20240902-00001-g131bf9439dd9 #69
[ 1.146821] Hardware name: NXP i.MX8ULP EVK (DT)
[ 1.151647] pstate: 40000005 (nZcv daif -PAN -UAO -TCO -DIT -SSBS BTYPE=--)
[ 1.158931] pc : nxp_fspi_exec_op+0xb04/0xb64
[ 1.163496] lr : nxp_fspi_exec_op+0xa34/0xb64
[ 1.168060] sp : ffff80008002b2a0
[ 1.171526] x29: ffff80008002b2d0 x28: 0000000000000000 x27: 0000000000000000
[ 1.179002] x26: ffff2eb645542580 x25: ffff800080610014 x24: ffff800080610000
[ 1.186480] x23: ffff2eb645548080 x22: 0000000000000006 x21: ffff2eb6455425e0
[ 1.193956] x20: 0000000000000000 x19: ffff80008002b5e0 x18: ffffffffffffffff
[ 1.201432] x17: ffff2eb644467508 x16: 0000000000000138 x15: 0000000000000002
[ 1.208907] x14: 0000000000000000 x13: ffff2eb6400d8080 x12: 00000000ffffff00
[ 1.216378] x11: 0000000000000000 x10: ffff2eb6400d8080 x9 : ffff2eb697adca80
[ 1.223850] x8 : ffff2eb697ad3cc0 x7 : 0000000100000000 x6 : 0000000000000001
[ 1.231324] x5 : 0000000000000000 x4 : 0000000000000000 x3 : 00000000000007a6
[ 1.238795] x2 : 0000000000000000 x1 : 00000000000001ce x0 : 00000000ffffff92
[ 1.246267] Call trace:
[ 1.248824] nxp_fspi_exec_op+0xb04/0xb64
[ 1.253031] spi_mem_exec_op+0x3a0/0x430
[ 1.257139] spi_nor_read_id+0x80/0xcc
[ 1.261065] spi_nor_scan+0x1ec/0xf10
[ 1.264901] spi_nor_probe+0x108/0x2fc
[ 1.268828] spi_mem_probe+0x6c/0xbc
[ 1.272574] spi_probe+0x84/0xe4
[ 1.275958] really_probe+0xbc/0x29c
[ 1.279713] __driver_probe_device+0x78/0x12c
[ 1.284277] driver_probe_device+0xd8/0x15c
[ 1.288660] __device_attach_driver+0xb8/0x134
[ 1.293316] bus_for_each_drv+0x88/0xe8
[ 1.297337] __device_attach+0xa0/0x190
[ 1.301353] device_initial_probe+0x14/0x20
[ 1.305734] bus_probe_device+0xac/0xb0
[ 1.309752] device_add+0x5d0/0x790
[ 1.313408] __spi_add_device+0x134/0x204
[ 1.317606] of_register_spi_device+0x3b4/0x590
[ 1.322348] spi_register_controller+0x47c/0x754
[ 1.327181] devm_spi_register_controller+0x4c/0xa4
[ 1.332289] nxp_fspi_probe+0x1cc/0x2b0
[ 1.336307] platform_probe+0x68/0xc4
[ 1.340145] really_probe+0xbc/0x29c
[ 1.343893] __driver_probe_device+0x78/0x12c
[ 1.348457] driver_probe_device+0xd8/0x15c
[ 1.352838] __driver_attach+0x90/0x19c
[ 1.356857] bus_for_each_dev+0x7c/0xdc
[ 1.360877] driver_attach+0x24/0x30
[ 1.364624] bus_add_driver+0xe4/0x208
[ 1.368552] driver_register+0x5c/0x124
[ 1.372573] __platform_driver_register+0x28/0x34
[ 1.377497] nxp_fspi_driver_init+0x1c/0x28
[ 1.381888] do_one_initcall+0x80/0x1c8
[ 1.385908] kernel_init_freeable+0x1c4/0x28c
[ 1.390472] kernel_init+0x20/0x1d8
[ 1.394138] ret_from_fork+0x10/0x20
[ 1.397885] ---[ end trace 0000000000000000 ]---
[ 1.407908] ------------[ cut here ]------------