firmware: arm_scmi: Support only one single system power device

In order to minimize SCMI platform fw-side complexity, only one single SCMI
platform should be in charge of SCMI SystemPower protocol communications
with the OSPM. Enforce the existence of one single unique device associated
with SystemPower protocol across any possible number of SCMI platforms, and
warn if a system tries to register different SystemPower devices from
multiple platforms.

Link: https://lore.kernel.org/r/20220704101933.2981635-2-cristian.marussi@arm.com


Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>