ARM: shmobile: r8a7740: Correct name of DT Ethernet clock

The preferred node name in DT for an Ethernet device is "ethernet".
"sh-eth" was used in preliminary and incomplete bindings.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>