regulator: Implement list_voltage for WM835x LDOs and DCDCs

Implement the recently added voltage step listing API for the WM835x
DCDCs and LDOs. DCDCs can use values up to 0x66, LDOs can use the full
range of values in the mask. Both masks are the lower bits of the
register.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>