regulator: support PMIC mc13892

add support for mc13892, tested on mx51 babbage board

Signed-off-by: Arnaud Patard <arnaud.patard@rtp-net.org>
Signed-off-by: Yong Shen <yong.shen@linaro.org>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Acked-by: Samuel Ortiz <sameo@linux.intel.com>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>