dt-bindings: mfd: Document the Xylon LogiCVC multi-function device

dt-bindings: mfd: Document the Xylon LogiCVC multi-function device

The LogiCVC is a display engine which also exposes GPIO functionality.
For this reason, it is described as a multi-function device that is expected
to provide register access to its children nodes for gpio and display.

Signed-off-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20191203141243.251058-3-paul.kocialkowski@bootlin.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>