arm64: dts: imx8mm-verdin: Do not power down eth-phy

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

arm64: dts: imx8mm-verdin: Do not power down eth-phy

Currently if suspending using either freeze or memory state, the fec
driver tries to power down the phy which leads to crash of the kernel
and non-responsible kernel with the following call trace:

[ 24.839889 ] Call trace:
[ 24.839892 ] phy_error+0x18/0x60
[ 24.839898 ] kszphy_handle_interrupt+0x6c/0x80
[ 24.839903 ] phy_interrupt+0x20/0x2c
[ 24.839909 ] irq_thread_fn+0x30/0xa0
[ 24.839919 ] irq_thread+0x178/0x2c0
[ 24.839925 ] kthread+0x154/0x160
[ 24.839932 ] ret_from_fork+0x10/0x20

Since there is currently no functionality in the phy subsystem to power
down phys let's just disable the feature of powering-down the ethernet
phy.