armv7m: use consistent arm.cpsr member

We already set cpsr in armv7m_build_reg_cache, so lets use it for all other
accesses to this field.

Change-Id: I19b3b21ecf1571bbea12e1be664845e6544f6fa1
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/1539
Tested-by: jenkins