dt-bindings: soc: fsl: qe: document new fsl,qe-snums binding

Reading table 4-30, and its footnotes, of the QUICC Engine Block
Reference Manual shows that the set of snum _values_ is not
necessarily just a function of the _number_ of snums, as given in the
fsl,qe-num-snums property.

As an alternative, to make it easier to add support for other variants
of the QUICC engine IP, this introduces a new binding fsl,qe-snums,
which automatically encodes both the number of snums and the actual
values to use.

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Li Yang <leoyang.li@nxp.com>