avr32: Enable pullup on USART TX lines

Make USART initialization conform to Section 24.6.1 in the AT32AP7000 manual,
i.e. "To prevent the TXD line from falling when the USART is disabled, the use
of an internal pull up is mandatory."

Signed-off-by: Anders Blomdell <anders.blomdell@control.lth.se>
[haavard.skinnemoen@atmel.com: enable pullup on RX as well]
Signed-off-by: Haavard Skinnemoen <haavard.skinnemoen@atmel.com>