The rdev print helpers are a nice way to print messages related to a
specific regulator device. Move them from core.c to internal.h

As the rdev print helpers use rdev_get_name() export it from core.c. Also
move the declaration from coupler.h to driver.h because the rdev name is
not just a coupled regulator property. I guess the main audience for
rdev_get_name() will be the regulator core and drivers.
Signed-off-by: NMatti Vaittinen <matti.vaittinen@fi.rohmeurope.com>
Link: https://lore.kernel.org/r/dc7fd70dc31de4d0e820b7646bb78eeb04f80735.1622628333.git.matti.vaittinen@fi.rohmeurope.comSigned-off-by: NMark Brown <broonie@kernel.org>