i2c: sh_mobile: Remove dummy runtime PM callbacks

Platform drivers don't need dummy runtime PM callbacks that just return
success and non-NULL pm pointer in their struct device_driver in order
to have runtime PM happening. This has changed since following commits:

05aa55dd ("PM / Runtime: Lenient generic runtime pm callbacks")
543f2503 ("PM / platform_bus: Allow runtime PM by default")
8b313a38

 ("PM / Platform: Use generic runtime PM callbacks directly")

Signed-off-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>