greybus: endo: rework some attributes

The SVC is not the same as the Endo.  There are some attributes
(such as the Endo ID) that are independent of attributes of
the SVC (like its version).

The current "Endo attributes" are really SVC attributes.
Rename a few functions and variables to reflect that.

Add a new attribute group for Endo-specific attributes, and
populate it with the Endo ID.

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