[ADC] Support measuring VCC on Raspberry Pico (W) (#5335)

* [ADC] Support measuring VCC on Raspberry Pico (W)

Added support for measuring VCC on Raspberry Pico (W) with ADC.
GPIO pin is provided as `VCC`, same as with ESP8266. VSYS is the voltage
being actually processed, and might have an offset from actual power
supply voltage (e.g. USB on VBUS) due to voltage drop on
Schottky diode between VSYS and VBUS on Rasberry Pico. The offset has
experimentally been found to be ~0.25V on Pico W and ~0.1 on Pico,
presumably due to different power consumption.

Example usage:

	sensor:
	  - platform: adc
	    pin: VCC
	    name: "VSYS"

* + Added tests for VCC measuring on `rpipicow` board

esphome/components/adc/__init__.py

@@ -5,6 +5,10 @@ from esphome.const import CONF_ANALOG, CONF_INPUT
 
 from esphome.core import CORE
 from esphome.components.esp32 import get_esp32_variant
+from esphome.const import (
+    PLATFORM_ESP8266,
+    PLATFORM_RP2040,
+)
 from esphome.components.esp32.const import (
     VARIANT_ESP32,
     VARIANT_ESP32C2,
@@ -143,7 +147,7 @@ ESP32_VARIANT_ADC2_PIN_TO_CHANNEL = {
 
 def validate_adc_pin(value):
     if str(value).upper() == "VCC":
-        return cv.only_on_esp8266("VCC")
+        return cv.only_on([PLATFORM_ESP8266, PLATFORM_RP2040])("VCC")
 
     if str(value).upper() == "TEMPERATURE":
         return cv.only_on_rp2040("TEMPERATURE")