leon3: use qemu_irq framework instead of callback as property

leon3: use qemu_irq framework instead of callback as property

"set_pin_in" property is used to define a callback mechanism where the
device says "call the callback function, passing it an opaque cookie
and a 32-bit value". We already have a generic mechanism for doing
that, which is the qemu_irq. So we should just use that.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: KONRAD Frederic <frederic.konrad@adacore.com>