arm-bsp-extras/tf-m: Fix RSE<->AP communication hang

Backport a patch from TF-M to fix a bug in which the MHU interrupt
occurs while the scheduler is locked. This causes the RSE communication
request from AP to be not handled by RSE, resulting in the hang of
the Application Processor indefinitely waiting for a reply from RSE.

Signed-off-by: Ziad Elhanafy <ziad.elhanafy@arm.com>