arm64: dts: juno: add mhu doorbell support and scmi device nodes

@Ionela.Voinescu, later edit: changed compatibility string from
arm,juno-scp-shmem to arm,scmi-shmem

Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>

# Conflicts:
#	arch/arm64/boot/dts/arm/juno-base.dtsi