dts: bindings: adc: Add RaspberryPi Pico ADC

Define RaspberryPi Pico ADC.

The ADC has internally connected temperature sensor,
Add property to enable this.

The ADC has a single VREF. VCC usually connects to it,
but it may not be in a case.
Add property to make configurable it.

Signed-off-by: TOKITA Hiroshi <tokita.hiroshi@fujitsu.com>