ARM: shmobile: r8a7779: cleanup registration of sh_eth

sh_eth driver which needs platform data at the time of
registration is not used.
Now, ARM/shmobile aims to support DT,
and the C code base board support will be removed
if DT support is completed.
Current driver registration method which needs platform data
and which is not shared complicates codes.
This means legacy C code cleanup after DT supporting
will be more complicated
This patch removes r8a7779_add_ether_device()

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>