regulators: Add definition of regulator_set_voltage_time() for !CONFIG_REGULATOR

We already have dummy implementation for most of the regulators APIs for
!CONFIG_REGULATOR case and were missing it for regulator_set_voltage_time().

Found this issue while compiling cpufreq-cpu0 driver without regulators support
in kernel.

drivers/cpufreq/cpufreq-cpu0.c: In function ‘cpu0_cpufreq_probe’:
drivers/cpufreq/cpufreq-cpu0.c:186:3: error: implicit declaration of function ‘regulator_set_voltage_time’ [-Werror=implicit-function-declaration]

Fix this by adding dummy definition for regulator_set_voltage_time().

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Mark Brown <broonie@linaro.org>