ISL6271A voltage regulator support.

This device is very simple, it supports one buck and two LDOs. The LDOs are
fixed-voltage. Only the buck is programable over the I2C bus to 16 possible
voltages.

Signed-off-by: Marek Vasut <marek.vasut@gmail.com>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>