media: dvb-frontends: add ST STV6111 DVB-S/S2 tuner frontend driver

This adds a frontend driver for the ST STV6111 DVB-S/S2 tuners. Like the
stv0910 demod frontend driver, this driver originates from the Digital
Devices' dddvb vendor driver package as of version 0.9.29, and was cleaned
up aswell. No functionality had to be removed though. Any camel case has
been converted to kernel_case, fixup patch has been proposed upstream.

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>