V4L/DVB: Add FE_CAN_PSK_8 to allow apps to identify PSK_8 capable DVB devices

The enum fe_caps provides flags that allow an application to detect
whether a device is capable of handling various modulation types etc.
A flag for detecting PSK_8, however, is missing.
This patch adds the flag FE_CAN_PSK_8 to frontend.h and implements
it for the gp8psk-fe.c and cx24116.c driver (apparently the only ones
with PSK_8). Only the gp8psk-fe.c has been explicitly tested, though.

Signed-off-by: Klaus Schmidinger <Klaus.Schmidinger@tvdr.de>
Tested-by: Derek Kelly <user.vdr@gmail.com>
Acked-by: Manu Abraham <manu@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>