cortex_a: Add support for A15 MPCore

cortex_a: Add support for A15 MPCore

Added Cortex-A15 support for DAP AHB-AP init code as per ADI V5 spec.
Also added changes to make the APB MEM-AP to work with A15.
Made the the cortex_a target code  generic to work with A8, A9
and A15 single core or multicore implementation. Added armv7a code
for os_border calculation to work for known A8, A9 and A15
platforms based on the ARM DDI 0344H, ARM DDI 0407F, ARM DDI 0406C
ARMV7A architecture docs.

Change-Id: Ib2803ab62588bf40f1ae4b9192b619af31525a1a
Signed-off-by: Kamal Dasu <kdasu.kdev@gmail.com>
Reviewed-on: http://openocd.zylin.com/1601
Tested-by: jenkins
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>