of: reserved_mem: set dma_ops for devices using reserved mem

For some IPs, there may be virtual child devices created and for them its
necessary to set the dma_ops if it's using reserved memory else it will call
the dummy dma_ops during buffer operations for the child devices which will
lead to memory mapping failure.

Signed-off-by: Smitha T Murthy <smitha.t@samsung.com>
Signed-off-by: Pankaj Dubey <pankaj.dubey@samsung.com>
Acked-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Rob Herring <robh@kernel.org>