drm_fb_cma_helper.c: Add module information

Allows it to be built as a module.
zynq.dtsi: Add devicetree entries for OCM and DDRC

Adds on-chip memory and DDR error detection capabilities.
Use dma_request_slave_channel() instead of_dma_request_slave_channel()

of_dma_request_slave_channel() is not exported for use in modules, so use
dma_request_slave_channel() instead.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>