greybus: rename "gpbridge.h"

The file "gpbridge.h" is now used as a single place to define
all protocol message structures.  These protocols are not
necessarily related to the GP bridge, so the name of the
file is misleading.

Rename it "greybus_protocols.h".

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