Platform: Musca-B1: document core switch to CPU1

Musca B1 now switches core execution from CPU0 to CPU1 due to
performance benefits. Document this in TF-M Musca B1 documentation.

Change-Id: I55f6523c52dc859e4599523558e0a27076eaeda6
Signed-off-by: Amjad Ouled-Ameur <amjad.ouled-ameur@arm.com>
1 file changed