firmware: arm_scmi: Add receive buffer support for notifications

With all the plumbing in place, let's just add the separate dedicated
receive buffers to handle notifications that can arrive asynchronously
from the platform firmware to OS.

Also add one check to see if the platform supports any receive channels
before allocating the receive buffers: since those buffers are optionally
supported though, the whole xfer initialization is also postponed to be
able to check for their existence in advance.

Link: https://lore.kernel.org/r/20200327143438.5382-2-cristian.marussi@arm.com


Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
[Changed parameters in __scmi_xfer_info_init()]
Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>