i2c: Add a sysfs interface to instantiate devices

Add a sysfs interface to instantiate and delete I2C devices. This is
primarily a replacement of the force_* module parameters implemented
by some i2c drivers. These module parameters were implemented
internally by the I2C_CLIENT_INSMOD* macros, which don't scale well.

This can also be used when developing a driver on a self-soldered
board which doesn't yet have proper I2C device declaration at the
platform level, and presumably for various debugging situations.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Cc: David Brownell <dbrownell@users.sourceforge.net>