dt-bindings: bus: imx-weim: document optional burst clock mode

An optional devicetree property was added to the imx-weim driver,
which if present instructs it to operate in burst clock mode.
Update the dt-bindings to reflect this.

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