common: board_f: Make reserve_mmu a weak function

Make reserve_mmu a weak so that it provides an option
to customize this routine as per platform need

Signed-off-by: Siva Durga Prasad Paladugu <sivadur@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Simon Glass <sjg@chromium.org>