gpio/pcf857x: Copy i2c_device_id from old pcf8574 driver

The deprecated pcf8574 driver is going to be removed. Make sure the
replacement driver inherits all i2c_device_ids for a smooth transition.

Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>
Cc: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Jean Delvare <khali@linux-fr.org>