hw/riscv: sifive_u: Simplify the GEM IRQ connect code a little bit

hw/riscv: sifive_u: Simplify the GEM IRQ connect code a little bit

There is no need to retrieve all PLIC IRQ information in order to
just connect the GEM IRQ. Use qdev_get_gpio_in() directly like
what is done for other peripherals.

Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 1591625864-31494-4-git-send-email-bmeng.cn@gmail.com
Message-Id: <1591625864-31494-4-git-send-email-bmeng.cn@gmail.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>