dt-bindings: simplefb: Drop the advice about using a specific path for nodes

This goes contrary to how devicetree usually works, so drop it. Instead if
the firmware needs to be able to find a specific node it should use a
platform specific compatible + properties for this.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Grant Likely <grant.likely@linaro.org>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>