avr32: add RTS/CTS/CLK pin selection for the USARTs

Adds extra parameter to AT32 at32_map_usart(), so as to reserve
RTS/CTS/CLK pins.

All boards under arch/avr32/boards have been updated (trivial change), but
not all have been tested.

Signed-off-by: Peter Ma <pma@mediamatech.com>
Signed-off-by: Haavard Skinnemoen <haavard.skinnemoen@atmel.com>