mfd: axp20x: Add bindings for AXP809 PMIC

This patch adds the basic and regulator bindings for the X-Powers AXP809
PMIC.

Also update the DC-DC converter operating frequency for AXP22X/AXP80X.

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