rockchip: rk3308: Implement checkboard() to print SoC variant

rockchip: rk3308: Implement checkboard() to print SoC variant

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

  SoC:   RK3308
  SoC:   RK3308B
  SoC:   RK3308B-S

when U-Boot proper is running.

  U-Boot 2025.01-rc1 (Nov 02 2024 - 20:26:25 +0000)

  Model: Radxa ROCK Pi S
  SoC:   RK3308B
  DRAM:  512 MiB (effective 510 MiB)

Information about the SoC variant is read from GRF.

Signed-off-by: Jonas Karlman <jonas@kwiboo.se>