ARM: dts: i.MX51: digi-connectcore-som: Correct Ethernet node name

make dtbs_check:

    lan9221@5,0: $nodename:0: 'lan9221@5,0' does not match '^ethernet(@.*)?$'

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>