armv7a: Improve parsing of MPIDR register to avoid error message for Cortex R5

armv7a: Improve parsing of MPIDR register to avoid error message for Cortex R5

References:
- ARM DDI0406C ARMv7 Architecture Reference Manual, section B4.1.106
- ARM DDI0460D Cortex-R5 Technical Reference Manual section 4.3.6
- ARM 100048_0002_0 Cortex-A73 Technical Reference Manual section 4.5.2

Tested on: TMS570LC4357

Change-Id: Ie0d45fb697697f78cc4ad4e7a0116be9772590ba
Signed-off-by: Tommy Vestermark <tov@vestermark.dk>
Reviewed-on: http://openocd.zylin.com/5108
Tested-by: jenkins
Reviewed-by: Matthias Welwarsky <matthias@welwarsky.de>