i2c-mv64xxxx: allow more than one driver instance

The driver currently checks the platform device id and rejects platform
device id different from 0. This prevents the registration of a second
i2c controller on systems where a second one might be available (such as
Kirkwood 88F6282).

CC: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Nicolas Schichan <nschichan@freebox.fr>
Signed-off-by: Florian Fainelli <ffainelli@freebox.fr>
Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>