dts: arm: stm32f7 declares sram0 as zephyr, memory-region

Declare the SRAM0 region as memory-region
for the stm32f745 serie. Will be included for the stm32f746
for the stm32f765 serie. Will be included for the stm32f767
for the stm32f722 serie. Will be included for the stm32f723

Signed-off-by: Francois Ramu <francois.ramu@st.com>