Bluetooth: mesh: access: Fix model relation register

Added offset to the model relation register for vendor
models to find correct model index
for Composition Data Page 1. The previous implementation used the
'mod_idx' from the 'bt_mesh_model' struct, which led to issues in the
model relation register due to SIG and vender models having the same
model index.
Modified existing functions related to the model relation
register to take in the offset.

Modified macros for determining if a model is a base- or
extending model.

Added check in 'add_items_to_page' to check whether the model relation
is an extension.

Signed-off-by: Håvard Reierstad <haavard.reierstad@nordicsemi.no>