Allow the board to remap actual USART peripheral devices to serial
devices by calling at32_map_usart(hw_id, serial_line). This ensures
that even though ATSTK1002 uses USART1 as the first serial port, it
will still have a ttyS0 device.

This also adds a board-specific early setup hook and moves the
at32_setup_serial_console() call there from the platform code.
Signed-off-by: Haavard Skinnemoen <hskinnemoen@atmel.com>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>