yocto/safety_island: Reduce the iperf UDP test bandwidth

The HIPC test runs on the fvp-rd-n2-automotive FVP, the simulation
platform. The test results depend on the performance of the
workstation which runs the FVP.
So, to match more workstations with different performance, the
network bandwidth in test case is set to the minimum value of 100K.

Issue-Id: SCM-5876
Signed-off-by: Jingzhao Ni <Jingzhao.Ni@arm.com>
Change-Id: I3772bf343e2ded08f82bab39f8a8e76d64c7e57e