mediatek: Map SRAM as secure and cached memory

This patch changes the mapping of SRAM from non-secure to secure.
Without this patch, mmu_config_range() can not work when MMU is
enabled. The new config is still in non-secure cache since TTB section
is allocated in SRAM which is mapped as non-secure.

BUG=b:80501386
TEST=Boots correctly on Kukui and Elm

Change-Id: Ia5b8716cfcca64d1d716a177225ea2f7ac2920a6
Signed-off-by: Tristan Shieh <tristan.shieh@mediatek.com>
Reviewed-on: https://review.coreboot.org/27974
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Hung-Te Lin <hungte@chromium.org>
Reviewed-by: Julius Werner <jwerner@chromium.org>

src/soc/mediatek/mt8173/mmu_operations.c

@@ -23,7 +23,7 @@
 
 void mtk_soc_after_dram(void)
 {
-	mmu_config_range(_dram_dma, _dram_dma_size, UNCACHED_MEM);
+	mmu_config_range(_dram_dma, _dram_dma_size, NONSECURE_UNCACHED_MEM);
 	mtk_mmu_disable_l2c_sram();
 }