regulator: add missing prototype for regulator_is_supported_voltage

avoids needs for CONFIG_REGULATOR in sdhci.c

Signed-off-by: Philip Rakity <prakity@nvidia.com>
Reviewed-by: Eric Miao <eric.y.miao@gmail.com>
Signed-off-by: Eric Miao <eric.y.miao@gmail.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>