Add the EtherAVB clock to the R8A7790 device tree.

Based on original patch by Mitsuhiro Kimura <mitsuhiro.kimura.kc@renesas.com>.
Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>