run_model.sh: Enable unbuffured outputs for UART on FVP

While using the FVP noticed not all the UART logs are redirecting
to the log file immediately. UART logs are being copied into the
log file only after certain buffer size filled in the FVP.

Setting the FVP parameter unbuffered_output for the UART ports
is casuing the UART logs to redirect/copy to local file immediately.

Change-Id: I51baf7ab4268edf42af67ad25b5298080a7e4731
Signed-off-by: Dilip Kota <dilip.kota@arm.com>