IPA v4.5 (GSI v2.5) supports a larger set of channel protocols, and
adds an additional field to hold the most-significant bits of the
protocol identifier on a channel.

Add an inline function that encodes the protocol (including the
extra bits for newer versions of IPA), and define some additional
protocols.  At this point we still use only GPI protocol.
Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>