media: dt-bindings: nxp,imx8-jpeg: Document optional SRAM support

media: dt-bindings: nxp,imx8-jpeg: Document optional SRAM support

Document optional phandle to mmio-sram, which can describe an SRAM
region used for descriptor storage instead of regular DRAM region.
Use of SRAM instead of DRAM for descriptor storage may improve bus
access pattern and performance.

Reviewed-by: Frank Li <Frank.Li@nxp.com>
Signed-off-by: Marek Vasut <marek.vasut@mailbox.org>
Signed-off-by: Nicolas Dufresne <nicolas.dufresne@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil+cisco@kernel.org>