regulator: Add missing dummy definition for regulator_list_voltage

regulator: Add missing dummy definition for regulator_list_voltage

Fixes a build break when CONFIG_REGULATOR is not selected.

e.g, on linux-next - 07102015:

drivers/clk/tegra/clk-dfll.c: In function ‘find_lut_index_for_rate’:
drivers/clk/tegra/clk-dfll.c:691:3: error: implicit declaration of function ‘regulator_list_voltage’ [-Werror=implicit-function-declaration]
    if (regulator_list_voltage(td->vdd_reg, td->i2c_lut[i]) == uv)
    ^
   CC      drivers/clocksource/mmio.o
   CC      fs/proc/softirqs.o
cc1: some warnings being treated as errors
make[3]: *** [drivers/clk/tegra/clk-dfll.o] Error 1
make[2]: *** [drivers/clk/tegra] Error 2
make[1]: *** [drivers/clk] Error 2
make[1]: *** Waiting for unfinished jobs....

This should be pushed to 4.2 as we have the issue in 4.2-rc1, just that
nobody uses it without the REGULATOR(yet).

Signed-off-by: Suzuki K. Poulose <suzuki.poulose@arm.com>
Signed-off-by: Mark Brown <broonie@kernel.org>