sh_eth: get R8A7740 support out of #ifdef

Get the R8A7740 code/data in the driver out of #ifdef by adding "r8a7740-gether"
to the platform driver's ID table.  Change the GEther platform device's name in
the ARM platform code accordingly.

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