spl: Add missing prototype for board_boot_order

Add missing prototype to fix the sparse warning, warning: no
previous prototype for 'board_boot_order' [-Wmissing-prototypes].

Signed-off-by: Algapally Santosh Sagar <santoshsagar.algapally@amd.com>
Signed-off-by: Ashok Reddy Soma <ashok.reddy.soma@amd.com>
Link: https://lore.kernel.org/r/20230301103334.1455-3-ashok.reddy.soma@amd.com


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