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 <[email protected]>
Reviewed-by: Alistair Francis <[email protected]>
Message-id: 1591625864[email protected]
Message-Id: <1591625864[email protected]>
Signed-off-by: Alistair Francis <[email protected]>