clk: ralink: mtmips: fix clocks probe order in oldest ralink SoCs

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

clk: ralink: mtmips: fix clocks probe order in oldest ralink SoCs

Base clocks are the first in being probed and are real dependencies of the
rest of fixed, factor and peripheral clocks. For old ralink SoCs RT2880,
RT305x and RT3883 'xtal' must be defined first since in any other case,
when fixed clocks are probed they are delayed until 'xtal' is probed so the
following warning appears:

WARNING: CPU: 0 PID: 0 at drivers/clk/ralink/clk-mtmips.c:499 rt3883_bus_recalc_rate+0x98/0x138
Modules linked in:
CPU: 0 PID: 0 Comm: swapper Not tainted 6.6.43 #0
Stack : 805e58d0 00000000 00000004 8004f950 00000000 00000004 00000000 00000000
80669c54 80830000 80700000 805ae570 80670068 00000001 80669bf8 00000000
00000000 00000000 805ae570 80669b38 00000020 804db7dc 00000000 00000000
203a6d6d 80669b78 80669e48 70617773 00000000 805ae570 00000000 00000009
00000000 00000001 00000004 00000001 00000000 00000000 83fe43b0 00000000
...
Call Trace:
[<800065d0>] show_stack+0x64/0xf4
[<804bca14>] dump_stack_lvl+0x38/0x60
[<800218ac>] __warn+0x94/0xe4
[<8002195c>] warn_slowpath_fmt+0x60/0x94
[<80259ff8>] rt3883_bus_recalc_rate+0x98/0x138
[<80254530>] __clk_register+0x568/0x688
[<80254838>] of_clk_hw_register+0x18/0x2c
[<8070b910>] rt2880_clk_of_clk_init_driver+0x18c/0x594
[<8070b628>] of_clk_init+0x1c0/0x23c
[<806fc448>] plat_time_init+0x58/0x18c
[<806fdaf0>] time_init+0x10/0x6c
[<806f9bc4>] start_kernel+0x458/0x67c

---[ end trace 0000000000000000 ]---

When this driver was mainlined we could not find any active users of old
ralink SoCs so we cannot perform any real tests for them. Now, one user
of a Belkin f9k1109 version 1 device which uses RT3883 SoC appeared and
reported some issues in openWRT:
- https://github.com/openwrt/openwrt/issues/16054

Thus, define a 'rt2880_xtal_recalc_rate()' just returning the expected
frequency 40Mhz and use it along the old ralink SoCs to have a correct
boot trace with no warnings and a working clock plan from the beggining.