hw/riscv: sifive_u: Hook a GPIO controller

hw/riscv: sifive_u: Hook a GPIO controller

SiFive FU540 SoC integrates a GPIO controller with 16 GPIO lines.
This hooks the exsiting SiFive GPIO model to the SoC, and adds its
device tree data as well.

Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 1591625864-31494-8-git-send-email-bmeng.cn@gmail.com
Message-Id: <1591625864-31494-8-git-send-email-bmeng.cn@gmail.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>