These are defined in ISO/IEC 14496-3:2009/PDAM 4 for 6.1 and 7.1.
It also defines another 7.1 layout with configuration 14, that one
is not added here for now.

11: 3/3.1    FC FL+FR BL+BR BC LFE
12: 3/2/2.1  FC FL+FR SiL+SiR BL+BR LFE

Signed-off-by: Luca Barbato <lu_zero@gentoo.org>