ARM: dts: stm32: enable adc on stm32mp15xx-dkx boards

ARM: dts: stm32: enable adc on stm32mp15xx-dkx boards

U-Boot enables ADC1&2 to support USB power measurement and ADC
calibration on STM32MP15x Disco boards.
When leaving U-boot the ADCs do not return to power down state
to keep ADC linear calibration available for kernel.
Enable ADC1&2 by default on STM32MP15xx-DKx boards to align kernel
DT with Uboot. This avoids to shutdown the ADCs VDDA, while the
ADCs are not in power down.

Use STM32 ADC generic bindings instead of legacy bindings on
STM32MP15xx-DKx boards.

The ADC pins on Arduino connector are not set by default.
These pins are added in A7 Disco example DTs only.

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