cortex_a: remove partnum magic from arp_examine and dbginit

cortex_a: remove partnum magic from arp_examine and dbginit

Depending on the Debug implementation the "OS Lock" feature might be
implemented or not. It is not actually depending on the part number of the
implemented ARM core but on the DBGOSLSR.OSLM bits. This patch removes
querying the part number and implements proper parsing of OSLM. Result is
a more generic approach that will work out-of-box on more devices.

Change-Id: I79e052869c2f9af1d7fdedef42faddb7292e7332
Signed-off-by: Matthias Welwarsky <matthias@welwarsky.de>
Reviewed-on: http://openocd.zylin.com/3213
Tested-by: jenkins
Reviewed-by: Paul Fertser <fercerpav@gmail.com>