Bluetooth: GATT: Add BT_GATT_PERM_NONE convenience value

If an attribute has no special access permissions (e.g. it's
notify-only) it would set the permissions to 0. Having a dedicated
enum value for this makes the code a bit more readable.

Change-Id: I0d3d8716a2544379353148735ba29b1a137f7173
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>