media: dvb-frontends: add ST STV0910 DVB-S/S2 demodulator frontend driver

This adds a multi frontend driver for the ST STV0910 DVB-S/S2 demodulator
frontends. The driver code originates from the Digital Devices' dddvb
vendor driver package as of version 0.9.29, and has been cleaned up from
core API usage which isn't supported yet in the kernel, and additionally
all obvious style issues have been resolved. All camel case and allcaps
have been converted to kernel_case and lowercase. Patches have been sent
to the vendor package maintainers to fix this aswell. Signal statistics
acquisition has been refactored to comply with standards.

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>
Tested-by: Richard Scobie <r.scobie@clear.net.nz>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>