cortex_a: fix virt2phys when mmu is disabled

cortex_a: fix virt2phys when mmu is disabled

When the MMU is not enabled on debug state entry, virt2phys cannot
perform a translation since it is unknown whether a valid MMU
configuration existed before. In this case, return the virtual
address as physical address.

Change-Id: I6f85a7a5dbc200be1a4b5badf10a1a717f1c79c0
Signed-off-by: Matthias Welwarsky <matthias.welwarsky@sysgo.com>
Reviewed-on: http://openocd.zylin.com/4480
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-by: Matthias Welwarsky <matthias@welwarsky.de>