regulator: use voltage number array in 88pm860x

A lot of condition comparision statements are used in original driver. These
statements are used to check the boundary of voltage numbers since voltage
number isn't linear.

Now use array of voltage numbers instead. Clean code with simpler way.

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