rockchip: px30/rk3326: Implement checkboard() to print SoC variant

rockchip: px30/rk3326: Implement checkboard() to print SoC variant

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

SoC:   PX30
SoC:   PX30S
SoC:   PX30K
SoC:   RK3326
SoC:   RK3326S

when U-Boot proper is running.

The information is read from the OTP and also the DDR_GRF. There's no
public information as far as I know about the layout and stored
information on OTP but this was provided by Rockchip themselves through
their support channel.

The OTP stores the information of whether the SoC is PX30K or something
else. To differentiate between PX30/RK3326 and PX30S/RK3326S, one needs
to read some undocumented bitfield in a DDR_GRF register as done in
vendor kernel,
c.f. https://github.com/armbian/linux-rockchip/blob/rk-6.1-rkr5.1/drivers/soc/rockchip/rockchip-cpuinfo.c#L118-L133.

I do not own a PX30S, nor RK3326/RK3326S so cannot test it works
properly.

Also add the OTP node to the pre-relocation phase of U-Boot proper so
that the SoC variant can be printed when DISPLAY_BOARDINFO is enabled.
This is not required if DISPLAY_BOARDINFO_LATE is enabled because this
happens after relocation. If both are enabled, then the SoC variant will
be printed twice in the boot log, e.g.:

U-Boot 2025.07-rc3-00014-g7cb731574ae6-dirty (May 28 2025 - 13:52:47 +0200)

Model: Theobroma Systems PX30-uQ7 SoM on Haikou devkit
SoC:   PX30  <---- due to DISPLAY_BOARDINFO
DRAM:  2 GiB
PMIC:  RK809 (on=0x40, off=0x00)
Core:  293 devices, 27 uclasses, devicetree: separate
MMC:   mmc@ff370000: 1, mmc@ff390000: 0
Loading Environment from MMC... Reading from MMC(1)... OK

In:    serial@ff030000
Out:   serial@ff030000
Err:   serial@ff030000
Model: Theobroma Systems PX30-uQ7 SoM on Haikou devkit
SoC:   PX30  <----- due to DISPLAY_BOARDINFO_LATE
Net:   eth0: ethernet@ff360000

Signed-off-by: Quentin Schulz <quentin.schulz@cherry.de>
Reviewed-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>