xtensa: enable HAVE_DMA_CONTIGUOUS

Enable HAVE_DMA_CONTIGUOUS, reserve contiguous memory at bootmem_init,
use dma_alloc_from_contiguous and dma_release_from_contiguous in
xtensa_dma_alloc/free.
This allows for big contiguous DMA buffer allocation from designated
area configured in the device tree.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>