mediatek: Share MMU operation code among similar SOCs

Refactor MMU operation code which will be reused among similar SOCs.

BUG=b:80501386
BRANCH=none
TEST=Boots correctly on Elm

Change-Id: Id8173da0a02e57e863263fcd89c91a9c089e8a0f
Signed-off-by: Tristan Shieh <tristan.shieh@mediatek.com>
Reviewed-on: https://review.coreboot.org/27349
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Julius Werner <jwerner@chromium.org>

src/soc/mediatek/mt8173/flash_controller.c

@@ -25,8 +25,8 @@
 #include <string.h>
 #include <symbols.h>
 #include <timer.h>
+#include <soc/symbols.h>
 #include <soc/flash_controller.h>
-#include <soc/mmu_operations.h>
 
 #define get_nth_byte(d, n)	((d >> (8 * n)) & 0xff)