i2c-dev: Use standard bus notification mechanism

Use the standard driver core mechanism to keep track of i2c adapters
present on the system: i2c_for_each_dev and a notifier. This will let
us deprecate and ultimately remove the legacy attach_adapter and
detach_adapter callbacks in i2c_driver.

Signed-off-by: Jean Delvare <khali@linux-fr.org>