riscv: Enable CMA support

riscv has selected HAVE_DMA_CONTIGUOUS, but doesn't call
dma_contiguous_reserve().  This calls dma_contiguous_reserve(), which
enables CMA.

Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>