target: cortex_a: do not create new register cache every reset

target: cortex_a: do not create new register cache every reset

Commit 68101e67ac16bdead3bd6d48cbe0a2bfd63aac02 introduced a
regression which resulted for ever-growing registers list (as output
by "reg" command), its contents were doubled every reset (actually,
every examination).

Change-Id: Ie3409c795160a2fc840a5e8a892928df0bcc0c57
Reported-by: Daniele Emancipato <daniele12457@hotmail.com>
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/3100
Tested-by: jenkins
Reviewed-by: Matthias Welwarsky <matthias@welwarsky.de>
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>