i2c: imx: improve code readability

Replace of_get_named_gpio_flags with of_get_named_gpio because
the latter has less parameters, which improves code readability.

Signed-off-by: Fugang Duan <B38611@freescale.com>
Signed-off-by: Gao Pan <b54642@freescale.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>