V4L/DVB (7223): Add support for the ISL6405 dual LNB supply chip

The chip can control 2 LNBs independently.
The driver distinguishes them by evaluating the MSB of the override_set
parameter of the isl6405_attach function.

Signed-off-by: Hartmut Hackmann <hartmut.hackmann@t-online.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>