ARM: shmobile: kzm9g-reference: add ethernet support

Add a DT node for the SMSC 9221 ethernet chip, found on kzm9g, to its
reference implementation.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>