hw/riscv: microchip_pfsoc: Hook GPIO controllers

hw/riscv: microchip_pfsoc: Hook GPIO controllers

Microchip PolarFire SoC integrates 3 GPIOs controllers. It seems
enough to create unimplemented devices to cover their register
spaces at this point.

With this commit, QEMU can boot to U-Boot (2nd stage bootloader)
all the way to the Linux shell login prompt, with a modified HSS
(1st stage bootloader).

For detailed instructions on how to create images for the Icicle
Kit board, please check QEMU RISC-V WiKi page at:
https://wiki.qemu.org/Documentation/Platforms/RISCV

Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <1598924352-89526-15-git-send-email-bmeng.cn@gmail.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>