hw/riscv/sifive_plic: Use gpios instead of irqs

Instead of creating the interrupt in lines with qemu_allocate_irq() use
qdev_init_gpio_in() as this gives us the ability to use the qdev*gpio*()
helpers later on.

Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Suggested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Michael Clark <mjc@sifive.com>