rockchip: rk356x: Implement checkboard() to print SoC variant

rockchip: rk356x: Implement checkboard() to print SoC variant

Implement checkboard() to print current SoC model used by a board,
e.g. one of:

  SoC:   RK3566
  SoC:   RK3566T
  SoC:   RK3568
  SoC:   RK3568B2
  SoC:   RK3568J

when U-Boot proper is running.

  U-Boot 2025.01-rc1 (Nov 10 2024 - 00:39:37 +0000)

  Model: Generic RK3566/RK3568
  SoC:   RK3568J
  DRAM:  8 GiB (effective 7.7 GiB)

Information about the SoC model and variant is read from OTP.

Also update rk356x-u-boot.dtsi to include OTP in U-Boot pre-reloc phase,
where checkboard() is called.

Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Tested-by: FUKAUMI Naoki <naoki@radxa.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>