riscv: Add asm/dma-mapping.h for DMA mappings

This patch adds asm/dma-mapping.h for Linux-like DMA mappings
APIs required by some of the drivers (such as, Cadance MACB
Ethernet driver).

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Alexander Graf <agraf@suse.de>
Reviewed-by: Lukas Auer <lukas.auer@aisec.fraunhofer.de>