i2c-at91 supports new-style i2c drivers

Make i2c-at91 register as i2c adapter zero (none of these chips seem to
have more than one TWI controllers) to let it kick in any board-specific
device declarations; also make it hotplug/coldplug.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Acked-by: Jean Delvare <khali@linux-fr.org>
Cc: Andrew Victor <andrew@sanpeople.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>