[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 */