regulator: Factor out regulator name pretty printing

Some of the regulator API functions have code to allow the machine
constraints to override the device supplied name for the regulator
in the constraints in order to help tie logging to supplies on the
board and disambiguate when there is more than one regulator chip
in the system. Factor this code out into a new rdev_get_name()
function and use it throughout the regulator API so that we always
use the same name.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>