mfd: axp20x: Add separate MFD cell for AXP223

The AXP223 shares most of its logic with the AXP221 but has some
differences for the VBUS power supply driver. Thus, to probe the driver
with the correct compatible, the AXP221 and the AXP223 now have separate
MFD cells.

AXP221 MFD cells are renamed from axp22x_cells to axp221_cells to avoid
confusion.

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