regulator: helper: Document ramp_delay parameter of regulator_set_ramp_delay_regmap()

regulator: helper: Document ramp_delay parameter of regulator_set_ramp_delay_regmap()

With W=1:

    drivers/regulator/helpers.c:947: warning: Function parameter or member 'ramp_delay' not described in 'regulator_set_ramp_delay_regmap'

Fix it by documenting the parameter.

Fixes: fb8fee9efdcf ("regulator: Add regmap helper for ramp-delay setting")
Signed-off-by: ChiYuan Huang <cy_huang@richtek.com>
Link: https://lore.kernel.org/r/1686881298-28333-1-git-send-email-cy_huang@richtek.com
Signed-off-by: Mark Brown <broonie@kernel.org>