This will allow the HCI UART to use a non-heap mp_irq_obj_t, which avoids needing to make a root pointer for it.
mp_irq_init() is useful when the IRQ object is allocated by the caller. The mp_irq_methods_t.init method is not used anywhere so has been removed. Signed-off-by: Damien George <damien@micropython.org>
Initial implementation of this is taken from the cc3200 port.