firmware: arm_scmi: Add support for notifications message processing

firmware: arm_scmi: Add support for notifications message processing

Add the mechanisms to distinguish notifications from delayed responses
and command responses. Also add support to properly fetch notification
messages upon reception. Notifications processing does not continue any
further after the fetch phase.

Link: https://lore.kernel.org/r/20200327143438.5382-5-cristian.marussi@arm.com
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
[Reworked/renamed scmi_handle_xfer_delayed_resp()]
Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>