ARM: at91: do not configure at91sam9g10 twi pio as open-drain

As indicated in the datasheet, TWD and TWCK must not be programmed as
open-drain.

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