arm64: defconfig: Enable REGULATOR_AXP20X

The Allwinner A64 SoC is paired with the X-Powers AXP803 PMIC over the
Reduced Serial Bus (RSB). The regulators of this PMIC supply all power
rails of the SoC and many external peripherals.

Enable the driver for the regulators of this PMIC.

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
[wens@csie.org: refined commit message]
Signed-off-by: Chen-Yu Tsai <wens@csie.org>