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>
6 files changed