i2c-stu300: Use struct dev_pm_ops for power management

Make the ST-Ericsson U300 I2C controller driver define its PM
callbacks through a struct dev_pm_ops object rather than by using
legacy PM hooks in struct platform_driver.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>