tuna: PowerHAL: Define constants for CPUfreq paths

Same CPUfreq paths are used many times in the code.
This patch defines constants for them.

Cherry-picked and adjusted for tuna from
http://omapzoom.org/?p=device/ti/blaze_tablet.git;a=commit;h=f5ee7ba155978bd9e547c079b521a1a249bdefab

Change-Id: I07519ad70a2f243c6fe68b26dc8b409e6223772e