powerpc/microwatt: Add Ethernet to device tree

The liteeth network device is used in the Microwatt soc.

Signed-off-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210826122653.3236867-2-joel@jms.id.au