greybus: hd: generalise cport allocation

Generalise CPort allocation by allowing host-device drivers to override
the default implementation.

Also pass the connection flags down the stack as such information is
needed for proper CPort allocation. Specifically, this will initially be
used to allow the camera driver to allocate the dedicated CDSI CPorts.

Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>