arm946e: cleanup C0.C cache type reg access

arm946e: cleanup C0.C cache type reg access

Cache type register C0.C is read-only, and display
hard core configuration information.
This information is unlikely be changed in runtime.

 - removed C0.C access when result is not used in
   arm946e_invalidate_dcache()
 - access C0.C only once per target, store result
   in cp15_cache_info field of target structure
 - fix cache index count calculation

Change-Id: I12bc4c967fdf07f54d755f2f2f42406c0ababc1a
Signed-off-by: Alexander Osipenko <sipych@gmail.com>
Reviewed-on: http://openocd.zylin.com/693
Tested-by: jenkins
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>