rockchip: rk3568-rock-3a: Enable boot from SPI NOR flash

Add sfc and flash node to device tree and config options to enable
support for booting from SPI NOR flash on Radxa ROCK 3 Model A.

Unlike prior generation SoCs the BootRom in RK3568 can read all data and
look for idbloader at 0x8000, same as on SD and eMMC.

Use the rksd format and modify the mkimage offset to generate a bootable
u-boot-rockchip-spi.bin that can be written to 0x0 of SPI NOR flash. The
FIT image is loaded from 0x60000.

  => sf probe
  SF: Detected mx25u12835f with page size 256 Bytes, erase size 4 KiB, total 16 MiB

  => load mmc 1:1 10000000 u-boot-rockchip-spi.bin
  1384448 bytes read in 119 ms (11.1 MiB/s)

  => sf update $fileaddr 0 $filesize
  device 0 offset 0x0, size 0x152000
  1179648 bytes written, 204800 bytes skipped in 9.901s, speed 143185 B/s

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