samples: drivers: adc: Add rpi_pico support
samples: drivers: adc: Add rpi_pico support

Add support for rpi_pico board to adc driver sample.

Signed-off-by: TOKITA Hiroshi <501187a43400793219de4d849513395616209606@fujitsu.com>

galak/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr,finikorg/zephyr,finikorg/zephyr