mfd: axp20x: Add binding documentation for AXP152 PMIC

Add devicetree binding documentation for the AXP152 PMIC, this is a
stripped down version of the AXP202 PMIC with the battery charging
function removed.

Signed-off-by: Michal Suchanek <hramrach@gmail.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>