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

Commit: e676253b19b2d269cccf67fdb1592120a0cd0676 [3/21] 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: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Ricardo Ribalda Delgado <ricardo.ribalda@gmail.com>
Signed-off-by: Chris Zankel <chris@zankel.net>