greybus: connection: add support for high-priority connections

greybus: connection: add support for high-priority connections

Add connection flag to indicate that a connection has high priority.

For the SVC and control connections this may be used by the host-device
driver as a hint to allocate dedicated DMA channels or to use polling to
avoid message congestion.

We also allow drivers to set this flag on their connections, even though
we currently have no use case for this (and the host-device driver is
again free to ignore it).

Note that this mechanism can be used to indicate also high-bandwidth
connections (e.g. wanting larger buffers or dedicated endpoints), but
that that should be done using a separate high-bandwidth flag rather
than overloading the high-priority flag.

Reviewed-by: Alex Elder <elder@linaro.org>
Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>