platforms: add support to wait for debugger connection

run_model scripts starts simulation directly after CADI/IRIS server
is started. New argument (-w) is added to run_model scripts to wait
for debugger connection.

Signed-off-by: Ibrahim Uslan <ibrahim.uslan@arm.com>
Change-Id: I011e4b896264b45ba0b87b17f51b535d840a0962