LP3971 PMIC regulator driver (updated and combined version)

This patch adds regulator drivers for National Semiconductors LP3971 PMIC.
This LP3971 PMIC controller has 3 DC/DC voltage converters and 5 low
drop-out (LDO) regulators. LP3971 PMIC controller uses I2C interface.

Reviewed-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>