dt-bindings: power: supply: axp20x_usb_power: add axp813 compatible

This adds the "x-powers,axp813-usb-power-supply" to the list of
compatibles for AXP20X VBUS power supply driver.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>