aarch64: fix debug entry from EL0

aarch64: fix debug entry from EL0

If we enter debug state from EL0, some registers are not accessible.
Temporarily move to EL1H and back to gain access. Also, fix
armv8_dpm_modeswitch() to not immediately restore the previous state
on elevating the privilege level.

Change-Id: Ic2a92109230ff4eb6834c00ef544397a5b7ad56a
Signed-off-by: Matthias Welwarsky <matthias.welwarsky@sysgo.com>
Reviewed-on: http://openocd.zylin.com/4461
Tested-by: jenkins
Reviewed-by: Matthias Welwarsky <matthias@welwarsky.de>