arm: xea: Add support for boot image source descriptor in SPL

We load two boot image source descriptor structures from last
two sectors in the SPI NOR flash and determine the boot source
for loading the kernel/DTB images, then adjust the boot order for
loading image from eMMC boot0 or boot1 partition.

Signed-off-by: Anatolij Gustschin <agust@denx.de>
Signed-off-by: Lukasz Majewski <lukma@denx.de>