powerpc/wii.dts: Add drive slot LED

The Wii has a blue LED in the disk drive slot, which is controlled via a
GPIO line. Add this LED to wii.dts, and mark it as a panic-indicator.

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