regulator: consumer: Supply missing prototypes for 3 core functions

regulator_suspend_enable(), regulator_suspend_disable() and
regulator_set_suspend_voltage() are all exported members of the
API, but are all missing prototypes.

Fixes the following W=1 warning(s):

 drivers/regulator/core.c:3805:5: warning: no previous prototype for ‘regulator_suspend_enable’ [-Wmissing-prototypes]
 3805 | int regulator_suspend_enable(struct regulator_dev *rdev,
 | ^~~~~~~~~~~~~~~~~~~~~~~~
 drivers/regulator/core.c:3812:5: warning: no previous prototype for ‘regulator_suspend_disable’ [-Wmissing-prototypes]
 3812 | int regulator_suspend_disable(struct regulator_dev *rdev,
 | ^~~~~~~~~~~~~~~~~~~~~~~~~
 drivers/regulator/core.c:3851:5: warning: no previous prototype for ‘regulator_set_suspend_voltage’ [-Wmissing-prototypes]
 3851 | int regulator_set_suspend_voltage(struct regulator *regulator, int min_uV,
 | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20200625163614.4001403-2-lee.jones@linaro.org
Signed-off-by: Mark Brown <broonie@kernel.org>