powerpc/opal-irqchip: Use interrupt names if present

Recent versions of OPAL can provide names for the various OPAL interrupts,
so let's use them. This also modernises the code that fetches the
interrupt array to use the helpers provided by the generic code instead
of hand-parsing the property.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
[mpe: Free irqs on error, check allocation of names, consolidate error
      handling, whitespace.]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>