ARM: shmobile: genmai: adapt dts to use native i2c driver

Activate the shiny new riic driver for i2c2. Tested by accessing the
eeprom on the genmai board.

Signed-off-by: Wolfram Sang <wsa@sang-engineering.com>
Acked-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>