i2c: Add driver suspend/resume/shutdown support

Driver model updates for the I2C core:

 - Add new suspend(), resume(), and shutdown() methods.  Use them in the
   standard driver model style; document them.

 - Minor doc updates to highlight zero-initialized fields in drivers, and
   the driver model accessors for "clientdata".

If any i2c drivers were previously using the old suspend/resume calls
in "struct driver", they were getting warning messages ... and will
now no longer work.  Other than that, this patch changes no behaviors;
and it lets I2C drivers use conventional PM and shutdown support.

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