axp20x support has been split into 2 parts, I2C and RSB interface
variants.

Update the MFD_AXP20X symbol for I2C support, and also enable
MFD_AXP20X_RSB to support RSB variants.

Keep MFD_AXP20X enabled for now, to ease migration for automated boot
farms while the patches are being merged. This should be removed in the
next release.
Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>