dt-bindings: Add device tree binding for X-Powers AXP803 PMIC

AXP803 is a PMIC produced by Shenzhen X-Powers, with either I2C or RSB
bus.

Add a compatible for it.

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