tty/serial: atmel: move atmel_serial header into driver directory

atmel_serial.h is only used by atmel_serial.c, so there's no need for
it to lie in include/linux.

Suggested-by: Joe Perches <joe@perches.com>
Signed-off-by: Richard Genoud <richard.genoud@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>