mfd: axp20x: Make AXP209/22x cells probe their ADC via DT

This makes AXP209 and AXP22x ADCs probe first via DT and then by
fallback via platform.
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>

drivers/mfd/axp20x.c

@@ -660,6 +660,7 @@ static struct mfd_cell axp20x_cells[] = {
 		.name		= "axp20x-regulator",
 	}, {
 		.name		= "axp20x-adc",
+		.of_compatible	= "x-powers,axp209-adc",
 	}, {
 		.name		= "axp20x-battery-power-supply",
 		.of_compatible	= "x-powers,axp209-battery-power-supply",
@@ -684,7 +685,8 @@ static struct mfd_cell axp221_cells[] = {
 	}, {
 		.name		= "axp20x-regulator",
 	}, {
-		.name		= "axp22x-adc"
+		.name		= "axp22x-adc",
+		.of_compatible	= "x-powers,axp221-adc",
 	}, {
 		.name		= "axp20x-ac-power-supply",
 		.of_compatible	= "x-powers,axp221-ac-power-supply",
@@ -708,6 +710,7 @@ static struct mfd_cell axp223_cells[] = {
 		.resources		= axp22x_pek_resources,
 	}, {
 		.name		= "axp22x-adc",
+		.of_compatible	= "x-powers,axp221-adc",
 	}, {
 		.name		= "axp20x-battery-power-supply",
 		.of_compatible	= "x-powers,axp221-battery-power-supply",