tty/serial: atmel: make the driver DT only

Now that AVR32 is gone, platform_data are not used to initialize the driver
anymore, remove that path from the driver. Also remove the now unused
struct atmel_uart_data.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Richard Genoud <richard.genoud@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>