ARM: dts: stm32: add fixed regulators to support usb on stm32mp135f-dk

Add VDD_USB and 3V3_SW regulators on stm32mp135f-dk. Temporary add them
as fixed regulators, waiting for full SCMI regulators support.

This is a precursor patch to enable USB support on STM32MP13:
- VDD_USB supplies the STM32MP13 USB internals
- 3V3_SW supplies various peripherals, including the onboard HUB.
Note: USB support requires these regulators to be enabled before
entering the kernel.

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