The shared memory node used for communication between the firmware and
the OS should be compatible with "arm,scmi-shmem". Add the check for the
same while parsing the node before fetching the memory regions.

Link: https://lore.kernel.org/r/20210602073851.1005607-2-sudeep.holla@arm.com
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Cristian Marussi <cristian.marussi@arm.com>
Cc: Florian Fainelli <f.fainelli@gmail.com>
Cc: Jim Quinlan <jim2101024@gmail.com>
Cc: Etienne Carriere <etienne.carriere@linaro.org>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Tested-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Cristian Marussi <cristian.marussi@arm.com>
Tested-by: Cristian Marussi <cristian.marussi@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>