dt-bindings: bus: imx-weim: add words about continuous bclk

Document continuous Burst Clock option. With this option Burst Clock, if
enabled, will output continuous clock, otherwise Burst Clock will output
clock only when necessary.

Signed-off-by: Ivan Bornyakov <i.bornyakov@metrotek.ru>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>