ARM: dts: stm32: STM32MP13x SoC exposes SCMI regulators

Updates STM32MP13x SoC DTSI file to define the SoC voltage regulators
exposed by OP-TEE SCMI service and remove the fixed regulator
abstraction previously used until OP-TEE OS firmware embeds the
service which it does since its release tag 3.22.0.

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>