common/board_f: Make reserve_mmu generic

Introduce arch_reserve_mmu to allow for architecture-specific reserve_mmu
routines. Also, define a weak nop stub for it.

Signed-off-by: Ovidiu Panait <ovpanait@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/arm/lib/cache.c b/arch/arm/lib/cache.c
index 3cbed60..44dde26 100644
--- a/arch/arm/lib/cache.c
+++ b/arch/arm/lib/cache.c
@@ -121,7 +121,7 @@
 }
 #endif
 
-__weak int reserve_mmu(void)
+int arch_reserve_mmu(void)
 {
 	return arm_reserve_mmu();
 }