stm32: Add SDRAM support for stm32f746 discovery board

This patch adds SDRAM support for stm32f746 discovery board.
This patch depends on previous patch.
This patch is based on STM32F4 and emcraft's[1].

[1]:  https://github.com/EmcraftSystems/u-boot

Signed-off-by: Toshifumi NISHINAGA <tnishinaga.dev@gmail.com>
diff --git a/arch/arm/cpu/armv7m/config.mk b/arch/arm/cpu/armv7m/config.mk
index 4a53006..db4660e 100644
--- a/arch/arm/cpu/armv7m/config.mk
+++ b/arch/arm/cpu/armv7m/config.mk
@@ -5,4 +5,4 @@
 # SPDX-License-Identifier:	GPL-2.0+
 #
 
-PLATFORM_CPPFLAGS += -march=armv7-m -mthumb
+PLATFORM_CPPFLAGS += -march=armv7-m -mthumb -mno-unaligned-access