dma-mapping: don't BUG when calling dma_map_resource on RAM

Use WARN_ON_ONCE to print a stack trace and return a proper error
code instead.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>