powerpc: wii.dts: Add interrupt-related properties to GPIO node

The Hollywood GPIO controller is connected to the Hollywood PIC (&PIC1)
at IRQs 10 and 11; IRQ 10 for GPIO lines that are configured for access
by the PPC, 11 for GPIO lines that are configured for access by the
ARM926.

Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>