ARM: dts: stm32: fix -Wall W=1 compilation in stm32mp157 pinctrl for mcan

Fix compilations warnings detected by -Wall W=1 compilation option:
- node has a unit name, but no reg property

Signed-off-by: Erwan Le Ray <erwan.leray@st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>