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>

include/common.h

@@ -286,6 +286,7 @@ void board_show_dram(phys_size_t size);
  */
 int arch_fixup_fdt(void *blob);
 
+int reserve_mmu(void);
 /* common/flash.c */
 void flash_perror (int);