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: Ib5856ae3c02d540e3a2d493b2cef012f4c93572f
Signed-off-by: Amjad Ouled-Ameur <amjad.ouled-ameur@arm.com>
1 file changed