regulator: core: update kernel documentation for regulator_desc

commit df367931 (regulator: core: Provide regmap get/set bypass
operations) introduced regulator_[gs]et_bypass_regmap

However structure documentation for regulator_desc needs an update.
./scripts/kernel-doc include/linux/regulator/driver.h >/dev/null
generates:
Warning(include/linux/regulator/driver.h:233): No description found for parameter 'bypass_reg'
Warning(include/linux/regulator/driver.h:233): No description found for parameter 'bypass_mask'

Cc: Liam Girdwood <lgirdwood@gmail.com>
Cc: Mark Brown <broonie@opensource.wolfsonmicro.com>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>