Add the EtherAVB clock to the R8A7794 device tree.

Based on the commit eaa870b3 ("ARM: shmobile: r8a7791: add EtherAVB
clock").
Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>