bindings: fsl-imx-sdma: Document 'fsl,sdma-event-remap' property

Document the 'fsl,sdma-event-remap' property and provide an example of
its usage.

Cc: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>