components/safety_island: Pin shell thread

This allows to work around degraded network performance in specific
scenarios when using the proper MHUv3 driver on Linux.

The degraded scenarios were:
  * tcp upload from Safety Island cluster 1 and 2 to Primary Compute
  * udp download to Safety Island cluster 1 and 2 from Primary Compute

This allows the tcp upload scenario to reach 160Mbps, from 25Mbps
without pinning.

Signed-off-by: Ambroise Vincent <ambroise.vincent@arm.com>