riscv: cpu: make riscv_cpu_probe to EVT_DM_POST_INIT_R callback

Since the Patch 55171aed, VisionFive2 booting has been broken [1].
VisionFive2 board requires to enable CONFIG_TIMER_EARLY but booting went
to panic from initr_dm_devices due to lack of a timer device.

- Error logs
initcall sequence 00000000fffd8d38 failed at call 00000000402185e4
(err=-19)

Thus, we need to move riscv_cpu_probe function in order to register
the timer earlier than initr_dm_devices.

Fixes: 7fe32b34 ("event: Convert arch_cpu_init_dm() to use events")
Cc: Simon Glass <sjg@chromium.org>
Cc: Bin Meng <bmeng.cn@gmail.com>
Signed-off-by: Chanho Park <chanho61.park@samsung.com>
Tested-by: Milan P. Stanić <mps@arvanta.net>
Tested-by: Roland Ruckerbauer <mail@ruabmbua.dev>
Tested-by: Roland Ruckerbauer <mail@ruabmbua.dev>