i2c: imx: convert to use managed functions

- convert to use devm_request_and_ioremap, devm_kzalloc, devm_clk_get,
  devm_request_irq.
- clean up unused variables.

Signed-off-by: Richard Zhao <richard.zhao@freescale.com>
Reviewed-by: Shubhrajyoti D <Shubhrajyoti@ti.com>
Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>