ARM: dts: stm32: fix memory nodes to match with DT validation tool

DT validation ("make dtbs_check") has shown that some memory nodes were not
correctly written. This commit fixes this kind of issue:

"stm32f746-disco.dt.yaml: /: memory: False schema does not allow
{'device_type': ['memory'], 'reg': [[3221225472, 8388608]]}"

Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>