media: dvb-frontends: MaxLinear MxL5xx DVB-S/S2 tuner-demodulator driver

This adds the frontend driver for the MaxLinear MxL5xx family of tuner-
demodulators, as used on Digital Devices MaxS4/8 four/eight-tuner cards.

The driver was picked from the dddvb vendor driver package and - judging
solely from the diff - has undergone a 100% rework:

 - Silly #define's used to pass multiple values to functions were
   expanded. This resulted in macro/register names not being usable
   anymore for such occurences, but makes the code WAY more read-,
   understand- and maintainable.
 - CamelCase was changed to kernel_case
 - All typedef were removed
 - Overall code style was fixed, besides >80char lines in _defs.h and
   _regs.h, checkpatch is happy.
 - Also, signal stat acquisition was made to comply with the DVB API
   ways to do these things.

Permission to reuse and mainline the driver code was formally granted by
Ralph Metzler <rjkm@metzlerbros.de>.

Signed-off-by: Daniel Scheller <d.scheller@gmx.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>