ARM: dts: stm32: Increase CPU core voltage on STM32MP13xx DHCOR SoM

The STM32MP13xx DHCOR DHSBC is populated with STM32MP13xx part capable
of 1 GHz operation, increase the CPU core voltage to 1.35 V to make
sure the SoC is stable even if the blobs unconditionally force the CPU
to 1 GHz operation.

It is not possible to make use of CPUfreq on the STM32MP13xx because
the SCMI protocol 0x13 is not implemented by upstream OpTee-OS which
is the SCMI provider.

Fixes: 6331bddc ("ARM: dts: stm32: Add support for STM32MP13xx DHCOR SoM and DHSBC board")
Signed-off-by: Marek Vasut <marex@denx.de>
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>