ASoC: dt-bindings: sun50i-codec-analog: Add headphone amp regulator supply

On the Allwinner A64 SoC, the audio codec has a built-in headphone
amplifier. This amplifier has a power supply separate from the rest of
the analog audio circuitry.

Add a regulator supply property to handle this.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Mark Brown <broonie@kernel.org>