mfd: Add twl6030 regulator subdevices

This patch adds initial support for creating twl6030 PMIC
specific voltage regulators in the twl mfd driver.

Board specific regulator configurations will have to be passed from
respective board files.

Signed-off-by: Rajendra Nayak <rnayak@ti.com>
Signed-off-by: Balaji T K <balajitk@ti.com>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Reviewed-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>