powerpc: wii.dts: Expose the OTP on this platform

This can be used by the newly-added nintendo-otp nvmem module.

Signed-off-by: Emmanuel Gil Peyrot <linkmauve@linkmauve.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210801073822.12452-5-linkmauve@linkmauve.fr