stm32mp1: pwr: use the last binding for pwr

Update the driver to use the latest binding from kernel v5.5-rc1:
no more use syscon or regmap to access to pwr register and
only one pwr_regulators node with the compatibility "st,stm32mp1,pwr-reg"
is available.

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