i2c-au1550: Convert to platform driver

Convert the i2c-au1550 bus driver to platform driver, and
register a platform device for the Alchemy Db/Pb series of
boards.

Signed-off-by: Manuel Lauss <mano@roarinelk.homelinux.net>
Signed-off-by: Jean Delvare <khali@linux-fr.org>