arm: mvebu: Add support for generating PEX kwbimage
Add a new Kconfig option CONFIG_MVEBU_SPL_BOOT_DEVICE_PEX which instruct
make to generate kwbimage with PEX header. This image is used for booting
from PCI Express device which is in the Root Complex mode.
Support is very simple, SPL after finishes DDR training returns back to the
BootROM (via CONFIG_SPL_BOOTROM_SUPPORT option) and BootROM then start
executing U-Boot proper.