sh_eth: Add support for r8a7790 SoC

This is a copy of support for r8a7778/9 with the .rmiimode mode bit
of struct sh_eth_cpu_data set.

Also update R8A7779 to R8A777x.

Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: David S. Miller <davem@davemloft.net>