ARM: dts: stm32: add SCMI PMIC regulators on stm32mp135f-dk board

Since OP-TEE release tag 3.22.0, OP-TEE SCMI service for STM32MP13x
SoC family exposes PMIC voltage regulators. This change defines
them in the platform DTS file and removes the fixed regulators that
were previously defined to abstract them before OP-TEE firmware was
ready.

Signed-off-by: Etienne Carriere <etienne.carriere@foss.st.com>
Signed-off-by: Pascal Paillet <p.paillet@foss.st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>