firmware: arm_scmi: Add priv parameter to scmi_rx_callback

Add a new opaque void *priv parameter to scmi_rx_callback which can be
optionally provided by the transport layer when invoking scmi_rx_callback
and that will be passed back to the transport layer in xfer->priv.

This can be used by transports that needs to keep track of their specific
data structures together with the valid xfers.

Link: https://lore.kernel.org/r/20210803131024.40280-15-cristian.marussi@arm.com


Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>