dt-bindings: bus: imx-weim: document multiple address ranges per child node

The imx-weim driver was patched to allow correct WEIM configuration
when multiple address ranges are used in a child node.
Update the dt-bindings to reflect this.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sven Van Asbroeck <TheSven73@googlemail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>