OMAP: PM debug: Add PRCM register dump support

Allows dumping out current register contents from the debug filesystem, and
also allows user to add arbitrary register save points into code. Current
register contents are available under debugfs at:

[debugfs]/pm_debug/registers/current

To add a save point, do following:

From module init (or somewhere before the save call, called only once):
  pm_dbg_init_regset(n); // n=1..4, allocates memory for dump area #n

From arbitrary code location:
  pm_dbg_regset_save(n); // n=1..4, saves registers to dump area #n

After this, the register dump can be seen under [debugfs]/pm_debug/registers/n

Signed-off-by: Tero Kristo <tero.kristo@nokia.com>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>