cpufreq: tegra: add regulator dependency for T124

This driver is the only one that calls regulator_sync_voltage(), but it
can currently be built with CONFIG_REGULATOR disabled, producing
this build error:

drivers/cpufreq/tegra124-cpufreq.c: In function 'tegra124_cpu_switch_to_pllx':
drivers/cpufreq/tegra124-cpufreq.c:68:2: error: implicit declaration of function 'regulator_sync_voltage' [-Werror=implicit-function-declaration]
  regulator_sync_voltage(priv->vdd_cpu_reg);

My first attempt was to implement a helper for this function
for regulator_sync_voltage, but Mark Brown explained:

   We don't do this for *all* regulator API functions - there's some where
   using them strongly suggests that there is actually a dependency on
   the regulator API.  This does seem like it might be falling into the
   specialist category [...]
   Looking at the code I'm pretty unclear on what the authors think the
   use of _sync_voltage() is doing in the first place so it may be even
   better to just remove the call.  It seems to have been included in the
   first commit so there's not changelog explaining things and there's
   no comment either.  I'd *expect* it to be a noop as far as I can see.

This adds the dependency to make the driver always build successfully
or not be enabled at all. Alternatively, we could investigate if the
driver should stop calling regulator_sync_voltage instead.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Acked-by: Jon Hunter <jonathanh@nvidia.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>