linux-firmware: add options for DVB firmware files

There are three firmware files that can be installed, each for
different devices.

For example, the DIB0700 firmware can be used by quite a few DVB
USB sticks based on this chipset (I know of at least two of them).

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>