scmi: Add devicetree transport properties

SCMI transport characteristics can be optionally described in the
devicetree to override the default build-time values.

Tested on: Arm Morello Board
Reviewed by: andrew
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D47422
Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>