scmi: Redefine max message payload size

The defined maximum payload size should not include header and status
field. Fix the definition accordingly, so that it matches the
definitions as can be found in the Linux SCMI stack.
The semantic of this define is important since it will then be possible
to override such values from devicetree, like in Linux.

Tested on: Arm Morello Board
Reviewed by: andrew
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D47421
Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>