cxl: Split afu_register_irqs() function

Split the afu_register_irqs() function so that different parts can
be useful elsewhere.

Signed-off-by: Michael Neuling <mikey@neuling.org>
Acked-by: Ian Munsie <imunsie@au1.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>