regulator: Add docbook for soft start

The docbook for these members is missing. Add them.

Warning(include/linux/regulator/machine.h:147): No description
found for parameter 'soft_start'
Warning(include/linux/regulator/driver.h:197): No description
found for parameter 'set_soft_start'

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Mark Brown <broonie@kernel.org>