cortex_m: Use the correct method to access registers

cortex_m: Use the correct method to access registers

Convert the DWT register store to use a byte array and fix the byte order
bug uncovered by that. Also fix an incorrect access of the PC value.

Change-Id: Idb5acab71bdf5a96895c358324b05c335e4d32ca
Signed-off-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Reviewed-on: http://openocd.zylin.com/2476
Tested-by: jenkins
Reviewed-by: Uwe Bonnes <bon@elektron.ikp.physik.tu-darmstadt.de>
Reviewed-by: Paul Fertser <fercerpav@gmail.com>