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().