mips/uapi: Add definition of TIOC[SG]RS485

Commit: e676253b (serial/8250: Add
support for RS485 IOCTLs), adds support for RS485 ioctls for 825_core on
all the archs. Unfortunaltely the definition of TIOCSRS485 and
TIOCGRS485 was missing on the ioctls.h file

Reported-by: Markos Chandras <markos.chandras@imgtec.com>
Signed-off-by: Ricardo Ribalda Delgado <ricardo.ribalda@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>