make mc13783 regulator code generic

move some common functions and micros of mc13783 regulaor driver to
a seperate file, which makes it possible for mc13892 to share code.

Signed-off-by: Yong Shen <yong.shen@linaro.org>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>