riscv: Add DMA 64-bit address support

dma_addr_t holds any valid DMA address. If the DMA API only uses 32/64-bit
addresses, dma_addr_t need only be 32/64 bits wide.

Signed-off-by: Padmarao Begari <padmarao.begari@microchip.com>
Reviewed-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Rick Chen <rick@andestech.com>