arm: imx6q: pcm058: Rework SPI NOR configuration

Enable CONFIG_SPL_DM_SPI_FLASH to be able to boot from SPI NOR,
modify the offset of U-boot proper in the SPI NOR, so the
difference in offset matches between SPL and U-boot matches that of
the SD Card, allowing u-boot-with-spl.imx to also be copied to SPI
NOR at an offset of 0x400. Update the README to reflect this
change.

Signed-off-by: Niel Fourie <lusus@denx.de>
Cc: Stefano Babic <sbabic@denx.de>