mfd: AXP20x: Add bindings documentation

Bindings documentation for the AXP20x driver. In this file also
sub-nodes are documented.

Signed-off-by: Carlo Caione <carlo@caione.org>
[wens@csie.org: clarify interrupt source for the axp PMIC]
[wens@csie.org: explain dcdc-workmode in detail and trim lines to 80 chars]
[wens@csie.org: make regulator supplies optional if using unregulated input]
[wens@csie.org: use cubieboard2 regulator nodes as example]
[wens@csie.org: x-powers,dcdc-workmode default changed to 'current hardware setting']
[wens@csie.org: reorganized regulator related properties into separate section.]
Signed-off-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>