i2c: i2c-imx: Adapt the clock name to the new clock framework

With the new i.mx clock framework the i2c clock is registered as:

clk_register_clkdev(clk[i2c1_ipg_gate], NULL, "imx-i2c.0")

So we do not need to pass "i2c_clk" string and can use NULL instead.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
[wsa: rebased on top of the devm-conversion]

Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>