uboot-imx

mach-stm32: Set MPU SDRAM size to 512MB for STM32F7/H7

This allows to boot all STM32F7 and STM32H7 boards independently
of the amount of embedded SDRAM.

Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
Reviewed-by: Vikas Manocha <vikas.manocha@st.com>