greybus: connection: Add sysfs 'ap_cport_id' file for connections

Its a very useful piece of information, i.e. the cport id of the AP to
which the cport of the module is connected, and is required lots of
times. It isn't known in advance as it is allocated at runtime.

This patch creates another file 'ap_cport_id', only for the connection
directories, which will give the cport id of the AP.

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