Update documentation according to the changes in the driver.

New attributes group tty is exposed and ttyGNSS, ttyGNSS2, ttyMAC and
ttyNMEA are moved to this group. Also, these attributes are no more
links to the devices but rather simple text files containing names of
tty devices.
Signed-off-by: Vadim Fedorenko <vadfed@meta.com>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>