regulator: axp20x: set supply names for AXP22X DC1SW/DC5LDO internally

The DC1SW and DC5LDO regulators in the AXP22X are internally chained
to DCDC1 and DCDC5, hence the names. The original bindings used the
parent regulator names for the supply regulator property.

Since they are internally connected, the relationship should not be
represented in the device tree, but handled internally by the driver.

This patch has the driver remember the regulator names for the parent
DCDC1/DCDC5, and use them as supply names for DC1SW/DC5LDO.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Mark Brown <broonie@kernel.org>