ARM: dts: i.MX51: Add one more I2C2 pinmux entry

This adds one more I2C2 alternate pinmux entry.
GPIO1_2 <=> SCL
GPIO1_3 <=> SDA

Signed-off-by: Alexander Shiyan <shc_work@mail.ru>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>