remoteproc: fix for "dma-mapping: remove the DMA_MEMORY_EXCLUSIVE flag"

The commit 82c5de0a ("dma-mapping: remove the DMA_MEMORY_EXCLUSIVE
flag") removed the "flags" parameter for dma_declare_coherent_memory().
Remove the parameter from the call in rproc_add_virtio_dev().

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
[bjorn: Extended commit message]
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>