pmic: stpmic1: support new prefix node name for regulator

The '_' character is discouraged in the node name, this patch adds the
new prefix of regulator subnode, with the '-' character, in STM32MP1 driver
to support the new  naming rule in Linux kernel device trees.

It is a preliminary patch before Linux device tree synchronization
for STMicroelectronics boards.

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