armv7m: use generic register core_cache

This removes the armv7m::core_cache and uses the generic arm::core_cache.

Change-Id: If854281b31486cea8be005008f6a71a691b4c208
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/968
Tested-by: jenkins
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
armv7m: use generic arm read/write_core_reg

Change-Id: I0c15acc1278d2972269d294078495e6b069c830b
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/969
Tested-by: jenkins
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>