Bluetooth: Mesh: Add specific number of keys & groups

Since Mesh Spec and Mesh model sayes, some model no need
groups and no need keys, such as Config Server, but the
current implementation, all model use same configuration,
which cause some ram ext comsume.

So that, change to specific way, but will consume more footprint size.

Signed-off-by: Lingao Meng <menglingao@xiaomi.com>