dt-bindings: iio: adc: Add AXP803 compatible

dt-bindings: iio: adc: Add AXP803 compatible

The AXP803 compatible was introduced recently with a fallback to the
AXP813, but it was never documented.

Cc: Jonathan Cameron <jic23@kernel.org>
Cc: Lars-Peter Clausen <lars@metafoo.de>
Cc: Peter Meerwald-Stadler <pmeerw@pmeerw.net>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Acked-by: Chen-Yu Tsai <wens@csie.org>
Acked-by: Jernej Skrabec <jernej.skrabec@siol.net>
Acked-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/20210114113538.1233933-2-maxime@cerno.tech