cortex-a: Fix "Detected core" number is always '0'

cortex-a: Fix "Detected core" number is always '0'

Problem
No matter what target->coreid is, it always shows
  Detected core 0 dbgbase: ...

In dap_lookup_cs_component(), it decreases the core index value to zero
in order to find the desired core.
The reference to coreidx is necessary considering "a device which has nested
ROM tables, with each core described in its own table." (by Paul Fertser).

Change-Id: I9b56d45d6edf6639e748a625ab27787f8e5a5776
Signed-off-by: Alamy Liu <alamy.liu@gmail.com>
Reviewed-on: http://openocd.zylin.com/2902
Tested-by: jenkins
Reviewed-by: Matthias Welwarsky <matthias@welwarsky.de>
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>