arm: at91: G45 TWI: remove open drain setting for twi function gpios

The G45 datasheets explicitly states that setting the open drain property
on peripheral function gpios is not allowed. (How about other A91 chips?)

Signed-off-by: Nikolaus Voss <n.voss@weinmann.de>
Tested-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Reviewed-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>