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

include/linux/regulator/driver.h

@@ -348,6 +348,7 @@ enum regulator_type {
  * @ramp_delay_table:	Table for mapping the regulator ramp-rate values. Values
  *			should be given in units of V/S (uV/uS). See the
  *			regulator_set_ramp_delay_regmap().
+ * @n_ramp_values:	number of elements at @ramp_delay_table.
  *
  * @enable_time: Time taken for initial enable of regulator (in uS).
  * @off_on_delay: guard time (in uS), before re-enabling a regulator