rockchip: rk3576: Implement checkboard() to print SoC variant

rockchip: rk3576: Implement checkboard() to print SoC variant

Implement checkboard() to print current SoC model used by a board when
U-Boot proper is running.

  U-Boot 2025.04 (Apr 22 2025 - 20:43:17 +0000)

  Model: Generic RK3576
  SoC:   RK3576
  DRAM:  8 GiB

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

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