sh_eth: add R8A77781 support

Add support for another ARM member of the R-Car family, R-Car M1A, also known as
R8A77781 -- it will share the code with previously added R8A77790.

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: David S. Miller <davem@davemloft.net>