microblaze: spl: add board_boot_order() implementation

Microblaze has three boot modes defined in microblaze/include/asm/spl.h,
but only booting from NOR flash is currently useable. Add a custom
board_boot_order() implementation so that RAM and SPI boot modes can also
be selected if the corresponding load-image support is present.

Signed-off-by: Ovidiu Panait <ovidiu.panait@windriver.com>
Link: https://lore.kernel.org/r/20211130163358.2531677-3-ovidiu.panait@windriver.com


Signed-off-by: Michal Simek <michal.simek@xilinx.com>