drivers: adc: Add ADC properties in Renesas RA ADC node

- Add "channel-available-mask" property in ADC node
to detect which channels are available to use

- Add "add-average-count" property in ADC node to chose
number of count of the addition or average mode

- Change the source code of ADC to match with 2 new properties.

Signed-off-by: Khoa Nguyen <khoa.nguyen.xh@renesas.com>