arm64: defconfig: Enable MFD_AXP20X_RSB

The Allwinner A64 SoC is paired with the X-Powers AXP803 PMIC over the
Reduced Serial Bus (RSB).

Enable the driver for 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>