media: ddbridge: support dummy tuners with 125MByte/s dummy data stream

The Octopus V3 and Octopus Mini devices support set up of a dummy tuner
mode on port 0 that will deliver a continuous data stream of 125MBytes
per second while raising IRQs and filling the DMA buffers, which comes
handy for some stress, PCIe link and IRQ handling testing. The dummy
frontend is registered using dvb_dummy_fe's QAM dummy frontend. Set
ddbridge.dummy_tuner to 1 to enable this on the supported cards.

Picked up from the upstream dddvb-0.9.33 release.

Signed-off-by: Daniel Scheller <d.scheller@gmx.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>