rockchip: rockpro64: Build u-boot-rockchip-spi.bin

Enable CONFIG_ROCKCHIP_SPI_IMAGE to build u-boot-rockchip-spi.bin.
Define CONFIG_SYS_SPI_U_BOOT_OFFS to write u-boot.itb at the expected
offset. Enable CONFIG_LTO to reduce size of SPL so that the mkimage
output fit before the 0x60000 offset in u-boot-rockchip-spi.bin.

  => sf probe
  SF: Detected gd25q128 with page size 256 Bytes, erase size 4 KiB, total 16 MiB
  => load mmc 1:1 10000000 u-boot-rockchip-spi.bin
  1442304 bytes read in 27 ms (50.9 MiB/s)
  => sf update $fileaddr 0 $filesize
  device 0 offset 0x0, size 0x160200
  1421824 bytes written, 20480 bytes skipped in 9.501s, speed 155432 B/s

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