powerpc/xive: Add a name to the IRQ domain

We hope one day to handle multiple irq_domain in the XIVE driver.
Start simple by setting the name using the DT node.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20201210171450.1933725-6-clg@kaod.org