arm11: initialise DPM and register cache before reading DSCR for the first time

arm11: initialise DPM and register cache before reading DSCR for the first time

When target was already halted during the initial examination,
arm11_check_init() was trying to read, store and interpret DSCR
contents before the DPM structure is initialised. This caused
a segfault like described on
http://sourceforge.net/apps/trac/openocd/ticket/65 .

This is a totally untested attempt to fix this issue.

Change-Id: I2fff115679a3f0023e7a88c749ccb5f045d6cf01
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/2043
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>