Migrate all ixp4xx devices to the bitbanging I2C bus driver utilizing
the arch-neutral GPIO API (linux/i2c-gpio.h).

Tested by the nslu2-linux and openwrt projects in public firmware releases.
Signed-off-by: Michael-Luke Jones <mlj28@cam.ac.uk>
Acked-by: Rod Whitby <rod@whitby.id.au>
Signed-off-by: Jean Delvare <khali@linux-fr.org>