ARM: dts: stm32m1: add reg for pll nodes

Fix the following DT dtc warnings for stm32mp1 boards:

Warning (unit_address_vs_reg): /soc/rcc@50000000/st,pll@0:
  node has a unit name, but no reg property
Warning (unit_address_vs_reg): /soc/rcc@50000000/st,pll@1:
  node has a unit name, but no reg property
Warning (unit_address_vs_reg): /soc/rcc@50000000/st,pll@2:
  node has a unit name, but no reg property
Warning (unit_address_vs_reg): /soc/rcc@50000000/st,pll@3:
  node has a unit name, but no reg property

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@st.com>