target/cortex_a: examination should be done every time it's asked for

target/cortex_a: examination should be done every time it's asked for

It was observed on AM437x that after every reset the target's debug
regions are unpowered. To be able to properly communicate with the
target and perform cortex_a init debug access after a reset event the
examination need to be performed every time, not just on OpenOCD
start.

Change-Id: Idf272e127ee88341e806ee00df154eade573451d
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/2723
Tested-by: jenkins
Reviewed-by: Felipe Balbi <balbi@ti.com>