mfd: axp20x: Update DT bindings with AXP22x compatibles

Add AXP221 to the list of supported devices.

Also replace any mention of AXP20x in the document with a
generic "PMIC".

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>