regulator: bd718xx: rename bd71837 to 718xx

rename bd71837-regulator.c to bd718x7-regulator.c to reflect the
fact that also BD71847 is now supported by the driver.

Signed-off-by: Matti Vaittinen <matti.vaittinen@fi.rohmeurope.com>
Signed-off-by: Mark Brown <broonie@kernel.org>