[media] lgdt3306a: support i2c mux for use by em28xx

Adds an i2c mux to the lgdt3306a demodulator.  This was done to support
the Hauppauge WinTV-dualHD 01595 USB TV tuner (em28xx), which utilizes two
si2157 tuners behind gate control.

Signed-off-by: Kevin Cheng <kcheng@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>