sh_eth: Remove obsolete r8a7740-gether platform_device_id entry

Since commit 1fa59bda ("ARM: shmobile: Remove legacy board code
for Armadillo-800 EVA"), r8a7740 is only supported in generic DT-only
ARM multi-platform builds.  The driver doesn't need to match platform
devices by name anymore, hence remove the corresponding
platform_device_id entry.

Protect r8a7740_data by #ifdef CONFIG_OF as it's now referenced on DT
platforms only. Move it to a more logical position, in front of the
r8a777x support, so we can have a single #ifdef covering all r7s* and
r8a* support soon. This requires moving a few helper functions, too.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: David S. Miller <davem@davemloft.net>