powerpc/configs/microwattt: Enable Liteeth

Liteeth is the network device used by Microwatt.

Signed-off-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210826122653.3236867-3-joel@jms.id.au