greybus: Add bundle descriptor type

A bundle corresponds to a device and a greybus driver binds to it. This patch
adds a type and descriptor for bundle.

This also shuffles the values of 'enum greybus_descriptor_type' to align
them with Greybus Specifications.

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