samples: rpi_pico2: Add ADC-related samples for rpi_pico2

The Raspberry Pi Pico 2's device is compatible with the existing Pico 1.
The build system requires a `<board>.overlay` file, but these use the
pre-processing to #include the sibling rpi_pico.overlay files rather
than duplicating the contents as an attempt to keep things DRY.

Tested locally.

Signed-off-by: Andrew Featherstone <andrew.featherstone@gmail.com>