powerpc/embedded6xx/flipper-pic: Don't match all IRQ domains

On the Wii, there is a secondary IRQ controller (hlwd-pic), so
flipper-pic's match operation should not be hardcoded to return 1.
In fact, the default matching logic is sufficient, and we can completely
omit flipper_pic_match.

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