arm: l2cc: doc: fix device tree example typo

The list of attributes above details the use of the 'filter-ranges'
property, but the example improperly used 'filter-latency'.  Make these
consistent by fixing up the example.

Signed-off-by: Josh Cartwright <josh.cartwright@ni.com>
Signed-off-by: Rob Herring <rob.herring@calxeda.com>