cpufreq: OMAP: scale voltage along with frequency

Use the regulator framework to get the voltage regulator associated
with the MPU voltage domain and use it to scale voltage along with
frequency.

While here, CONFIG_CPU_FREQ_DEBUG doesn't exist anymore, so move
debug prints to use dev_dbg().

Special thanks to Afzal Mohammed for suggestions on more robust error
checking.

Cc: Afzal Mohammed  <afzal@ti.com>
Signed-off-by: Kevin Hilman <khilman@ti.com>