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/common/board_f.c b/common/board_f.c
index 26309c4..5c650f0 100644
--- a/common/board_f.c
+++ b/common/board_f.c
@@ -385,6 +385,11 @@
 	return 0;
 }
 
+__weak int arch_reserve_mmu(void)
+{
+	return 0;
+}
+
 static int reserve_video(void)
 {
 #ifdef CONFIG_DM_VIDEO
@@ -951,9 +956,7 @@
 	reserve_pram,
 #endif
 	reserve_round_4k,
-#ifdef CONFIG_ARM
-	reserve_mmu,
-#endif
+	arch_reserve_mmu,
 	reserve_video,
 	reserve_trace,
 	reserve_uboot,