soc: mediatek: Refactor sram control

Put sram enable and disable control in separate functions.

Signed-off-by: Weiyi Lu <weiyi.lu@mediatek.com>
Reviewed-by: Nicolas Boichat <drinkcat@chromium.org>
[mb: fix coding style of reading register and changing the read value]
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>