fixup! irqchip: irq-bcm2712-mip: Support for 2712's MIP

Use irq_domain_add_hierarchy so that the root pointer is initialised
correctly. Failure to do so leads to (at least) lockdep warnings when
they are enabled.

See: https://github.com/raspberrypi/linux/issues/5866



Signed-off-by: Phil Elwell <phil@raspberrypi.com>