powerpc/powermac: constify device_node in of_irq_parse_oldworld()

The of_irq_parse_oldworld() does not modify passed device_node so make
it a pointer to const for safety.  Drop the extern while modifying the
line.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210924105653.46963-2-krzysztof.kozlowski@canonical.com