sh_eth: add R8A7791 support

Add support for yet another ARM member of the R-Car family, R-Car M2, also known
as R8A7791 -- it will share the code and data with previously added R8A7790.
Despite the Ether devices in these SoCs are indistinguishable at least from the
driver's point of view, we do introduce a new platform device ID "r8a7791-ether"
unlike the wildcard ID used for R8A7778/9 SoCs, due to newly established policy
for the Renesas SoCs.

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