hw/riscv: microchip_pfsoc: Connect a DMA controller

On the Icicle Kit board, the HSS firmware utilizes the on-chip DMA
controller to move the 2nd stage bootloader in the system memory.
Let's connect a DMA controller to Microchip PolarFire SoC.

Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <1598924352-89526-11-git-send-email-bmeng.cn@gmail.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>