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: NFabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: NMark Brown <broonie@kernel.org>