regulator: support list voltage in da903x

Make da903x driver to list voltage and count voltage.

Signed-off-by: Haojian Zhuang <haojian.zhuang@marvell.com>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Eric Miao <eric.y.miao@gmail.com>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>