cortex_a: don't wait for target halted in deassert_reset()

cortex_a: don't wait for target halted in deassert_reset()

The tcl script src/target/startup.tcl has already the proper
centralized support to wait for all targets to halt after the
command "reset halt". The extra wait in cortex_a_deassert_reset()
is not required.
This extra wait is also an issue for multi-core support, because
waiting for one core to halt can delay the halt request to the
other cores.

Replace the indirect call to cortex_a_halt(), that embeds the wait
for halt, with a low-level halt sequence.

The on-going work on the reset framework is compatible with this
change; in fact it keeps in startup.tcl the wait for targets to
halt, even if current code proposal for cortex_a simply removes
the function cortex_a_deassert_reset().

Change-Id: Ic661c3791a29ba7d520e31f85a61f939a646feb5
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5472
Tested-by: jenkins
Reviewed-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>