regulator: fix a kernel-doc warning

document n_ramp_values field at struct regulator_desc, in order
to solve this warning:

	include/linux/regulator/driver.h:434: warning: Function parameter or member 'n_ramp_values' not described in 'regulator_desc'

Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>
Link: https://lore.kernel.org/r/15efc16e878aa327aa2769023bcdf959a795f41d.1656409369.git.mchehab@kernel.org


Signed-off-by: Mark Brown <broonie@kernel.org>