powerpc: Add ioctls for RS485 mode control of serial drivers

These ioctls take a struct serial_rs485
(see linux/serial.h) as argument. They are already available
on x86. This patch adds them for the powerpc architecture.

Signed-off-by: Matthias Fuchs <mfuchs@ma-fu.de>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>