ASoC: sgtl500: Document the required supplies

sgtl5000 has two required supplies: VDDA and VDDIO and one optional supply:
VDDD, so document this properly.

Not passing VDDA and VDDIO prevents the driver to probe successfully.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
Merge branch 'for-next' of git://git.kernel.org/pub/scm/linux/kernel/git/broonie/sound into mx6-audio-3.18

Conflicts:
	Documentation/devicetree/bindings/sound/sgtl5000.txt