ARM: shmobile: R8A7778: add Ether support

Add Ether clock and platform device for R8A7778 SoC; add a function to register
this device with board-specific platform data.

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Acked-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>