hw/dma: Add SiFive platform DMA controller emulation

hw/dma: Add SiFive platform DMA controller emulation

Microchip PolarFire SoC integrates a DMA engine that supports:
* Independent concurrent DMA transfers using 4 DMA channels
* Generation of interrupts on various conditions during execution
which is actually an IP reused from the SiFive FU540 chip.

This creates a model to support both polling and interrupt modes.

Signed-off-by: Bin Meng <bin.meng@windriver.com>
Acked-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <1598924352-89526-10-git-send-email-bmeng.cn@gmail.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>