ARM: dts: at91: sama7g5: add io-channel-cells to adc node

Add io-channel-cell to ADC node. It is necessary for DT users of ADC.

Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Link: https://lore.kernel.org/r/20221026124114.985876-4-claudiu.beznea@microchip.com