cortex_a: target implementation renames cortex_a8 to cortex_a

cortex_a: target implementation renames cortex_a8 to cortex_a

A previous commit changes the target name used by tcl scripts.
commit d9ba56c295f057e716519a798bf9cdb4898c24f4
target: rename cortex_a8 to cortex_a

The current change renames target functions and definitions in the
implementation from cortex_a8 to cortex_a.
This prepares the implementation to support Cortex-A8, A9, A15-MPCore
in one place.

Change-Id: I73b5a38a92c12ba5bd3b806fbbb664817575a6d7
Signed-off-by: Kamal Dasu <kdasu.kdev@gmail.com>
Reviewed-on: http://openocd.zylin.com/1599
Tested-by: jenkins
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>