i2c: imx: use struct representing i2c clk{div, val} pair

using struct representing the i2c clk{div, val} pair would
make the i2c_clk_div array more clear.

Signed-off-by: Jingchang Lu <b35083@freescale.com>
Reviewed-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>