powerpc: wii.dts: Add GPIO keys

The Wii has POWER and EJECT buttons, which are connected through
normalization logic to the GPIO controller (the length of an assertion
of these signals is always the same, regardless of how long the user
pressed the buttons).

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