mfd: axp20x: Add MFD cells for AXP20X and AXP22X battery driver

The X-Powers AXP20X and AXP22X PMICs can have a battery as power supply.

This patch adds the AXP20X/AXP22X battery driver to the MFD cells of the
AXP209, AXP221 and AXP223 MFD.

Signed-off-by: Quentin Schulz <quentin.schulz@free-electrons.com>
Acked-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>