hw/riscv: sifive_gpio: Clean up the codes

hw/riscv: sifive_gpio: Clean up the codes

Do various minor clean-ups to the exisiting codes for:

- coding convention conformance
- remove unnecessary blank lines
- spell SiFive correctly

Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 1591625864-31494-6-git-send-email-bmeng.cn@gmail.com
Message-Id: <1591625864-31494-6-git-send-email-bmeng.cn@gmail.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>