mfd: axp20x: Probe axp20x_adc driver for AXP813

This makes the axp20x_adc driver probe with platform device id
"axp813-adc".

Signed-off-by: Quentin Schulz <quentin.schulz@bootlin.com>
Acked-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>