[AVR32] ngw100 i2c-gpio tweaks

Make the NGW100 bitbang i2c use open drain signaling.

Also, speed it up, so it's closer to 100 kHz ... the code paths seem
to be long enough that the udelay isn't dominating bit times.  The
peak bit rate I observed was around 125 kHz, but that's with large
delays (usually before ACK/NAK) which hold the overall rate down to
around 80 kHz (call it 100 usec/byte on average).

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Haavard Skinnemoen <hskinnemoen@atmel.com>