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

src/target/arm.h

@@ -97,7 +97,7 @@ struct arm {
 	/** Handle to the PC; valid in all core modes. */
 	struct reg *pc;
 
-	/** Handle to the CPSR; valid in all core modes. */
+	/** Handle to the CPSR/xPSR; valid in all core modes. */
 	struct reg *cpsr;
 
 	/** Handle to the SPSR; valid only in core modes with an SPSR. */