dt-bindings: mfd: axp20x: Add AXP806 to supported list of chips

The binding already lists compatibles and regulators for the AXP806,
but it is missing from the list of supported chips at the beginning.

Add it.

Fixes: 204ae296 ("mfd: axp20x: Add bindings for AXP806 PMIC")
Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>