mfd: Rename twl4030_ routines in twl-regulator.c

This patch renames all twl4030_ functions to twl so that regulator driver
can be reused by Triton - TWL4030 and Phoenix - TWL6030.

Signed-off-by: Rajendra Nayak <rnayak@ti.com>
Signed-off-by: Balaji T K <balajitk@ti.com>
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Acked-by: Kevin Hilman <khilman@deeprootsystems.com>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>