firmware: arm_scmi: Check shmem property for channel availablity

Instead of declaring the channel availabilty unconditionally, let us
check for the presence of "shmem" property and return the channel
availablity accordingly.

Link: https://lore.kernel.org/r/20200327163654.13389-4-sudeep.holla@arm.com


Tested-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>