powerpc/pci: Use of_irq_parse_and_map_pci() helper

Instead of calling both of_irq_parse_pci() and irq_create_of_mapping(),
call of_irq_parse_and_map_pci(), which does the same thing. This will allow
making of_irq_parse_pci() a private, static function.

This changes the logic slightly in that the fallback path will also be
taken if irq_create_of_mapping() fails internally.

Signed-off-by: Rob Herring <robh@kernel.org>
[bhelgaas: fold in virq init from Stephen Rothwell <sfr@canb.auug.org.au>]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Michael Ellerman <mpe@ellerman.id.au>