media: dvb: add DVB-C2 and DVB-S2X parameter values

Extend the DVB frontend parameter enums with additional values specified
by the DVB-C2 (ETSI EN 302 769) and DVB-S2X (ETSI EN 302 307-2)
standards to be ready for frontend drivers for such receivers.

While most parameters will be "read-only" due to being autodetected by
the receiver and only being reported back for informational purposes,
the addition of SYS_DVBC2 to the delivery systems enum is required,
because there are DVB-C2 capable receivers which are not capable of
DVB-C/C2 autodetection and thus need this enum value to be explicitly
instructed to search for a DVB-C2 signal.

As for DVB-S2X, as that is an extension to DVB-S2, the same delivery
system enum as for DVB-S2 can be used.

Add the additional enum values and comments to the documentation.

Link: https://lore.kernel.org/linux-media/trinity-1b7c5a66-85d4-4595-a690-0fde965d49b3-1642146228587@3c-app-gmx-bap69


Signed-off-by: Robert Schlabbach <robert_s@gmx.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>