[riscv] Provide a DMA API implementation for RISC-V bare-metal systems

Provide an implementation of dma_map() that performs cache clean or
invalidation as required, and an implementation of dma_alloc() that
returns virtual addresses within the coherent mapping of the 32-bit
physical address space.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/include/bits/dma.h

@@ -0,0 +1,15 @@
+#ifndef _BITS_DMA_H
+#define _BITS_DMA_H
+
+/** @file
+ *
+ * Dummy architecture-specific DMA API implementations
+ *
+ * This file is included only if the architecture does not provide its
+ * own version of this file.
+ *
+ */
+
+FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
+
+#endif /* _BITS_DMA_H */

src/include/ipxe/dma.h

@@ -345,6 +345,9 @@ DMAAPI_INLINE ( op, dma ) ( struct dma_mapping *map, void *addr ) {
 	return ( virt_to_phys ( addr ) + map->offset );
 }
 
+/* Include all architecture-dependent DMA API headers */
+#include <bits/dma.h>
+
 /**
  * Map buffer for DMA
  *