ARM: dts: stm32: add dummy vdd_adc regulator on stm32mp135f-dk

Add a dummy fixed regulator vdd-adc for STM32 ADC,
on STM32MP135F-DK board, while SCMI regulators are not available.
This patch will have to be removed when SCMI regulator support
is added to STM32MP13.

This patch intends to allow ADC enabling on STM32MP13.
With this patch the ADC can probe but it cannot return
valid conversion data, as it's regulator is not actually enabled.

Signed-off-by: Olivier Moysan <olivier.moysan@foss.st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>