kernel: lantiq: ltq-vmmc: introduce user group for vmmc

kernel: lantiq: ltq-vmmc: introduce user group for vmmc

asterisk-chan-lantiq is by now the only user of the VMMC interface.
And asterisk runs as user 'asterisk' which doesn't give it permission
to open the /dev/vmmc* devices.
Introduce a new user group 'vmmc' and give permission to access the
/dev/vmmc* devices to that group.
Another commit for asterisk-chan-lantiq will add the 'asterisk' user
to that group.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
(cherry picked from commit 37bbed6f95856a507d727ffc863b20f8a8b35332)