ARM: dts: stm32: add PWR fixed regulators on stm32mp131

Add 1v1, 1v8 and 3v3 PWR regulators on stm32mp131. Temporary add them
as fixed regulators, waiting for full SCMI regulators support.

This is a precursor patch to enable USB support on STM32MP13.
Note: USB support requires these regulators to be enabled before
entering kernel.

Signed-off-by: Fabrice Gasnier <fabrice.gasnier@foss.st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>