firmware: arm_scmi: Add notifications support in transport layer

Add common transport-layer methods to:
 - fetch a notification instead of a response
 - clear a pending notification

Add also all the needed support in mailbox/shmem transports.

Link: https://lore.kernel.org/r/20200327143438.5382-4-cristian.marussi@arm.com
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>