ARM: shmobile: r8a7778: cleanup registration of i2c

i2c-rcar driver which doesn't need platform data at the time of
registration can be registerd on SoC.
And, registering these drivers in the SoC code can avoid
unwanted device numbering issue.
(ex. the i2c3 device number will be i2c.0 if i2c3 only registered)
This patch registers it on SoC code as cleanup C code

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