remoteproc: don't allow modular build

Remoteproc started using dma_declare_coherent_memory recently, which is
a bad idea from drivers, and the maintainers agreed to fix that.  But
until that is fixed only allow building the driver built in so that we
can remove the dma_declare_coherent_memory export and prevent other
drivers from "accidentally" using it like remoteproc.  Note that the
driver would also leak the declared coherent memory on unload if it
actually was built as a module at the moment.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>