powerpc/xive: Add a debug_show handler to the XIVE irq_domain

Full state of the Linux interrupt descriptors can be dumped under
debugfs when compiled with CONFIG_GENERIC_IRQ_DEBUGFS. Add support for
the XIVE interrupt controller.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20201210171450.1933725-7-clg@kaod.org