ARM: dts: stm32: Fix DTC warnings for stm32mp157

Fix DTC warnings for stm32mp157:

Warning (unit_address_vs_reg): /soc/pin-controller: node has a reg or ranges property, but no unit name
Warning (unit_address_vs_reg): /soc/pin-controller/uart4@0: node has a unit name, but no reg property
Warning (unit_address_vs_reg): /soc/pin-controller-z: node has a reg or ranges property, but no unit name
Warning (unit_address_vs_reg): /memory: node has a reg or ranges property, but no unit name

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