ASoC: dt-bindings: add regulator property to simple amplifier

Amplifier may have associated regulator, so add a property for it.

Signed-off-by: Vasily Khoruzhick <anarsoul@gmail.com>
Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Mark Brown <broonie@kernel.org>