boards: arm: atsamd21_xpro: Change sercom0 tx pad and control.

According to the documentation and pinctrl file, sercom0 tx should
be available on PA10C/SERCOM_PAD[2]. However, and somewhat confusingly,
txpo 2 meant SERCOM_PAD[0] with RTS/CTS flow control.

Changing this to txpo 1 uses SERCOM_PAD[2], which allows sercom0
to work as documented.

Signed-off-by: Tom Rothamel <tom@rothamel.us>