ARM: shmobile: r8a7791: add EtherAVB clock

Add the EtherAVB clock to the R8A7791 device tree.

Based on the commit 63d2d750 ("ARM: shmobile: r8a7790: add EtherAVB
clocks").

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>