sh_eth: Remove obsolete r8a777x-ether platform_device_id entry

Since commit 3d7608e4 ("ARM: shmobile: bockw: remove legacy
board file and config"), R-Car Gen1 SoCs are 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 sh_eth_set_rate_r8a777x() and r8a777x_data by #ifdef CONFIG_OF,
as they're now referenced on DT platforms only.

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>