V4L/DVB (3762): Add sysfs device links to dvb devices

Currently in /sys/class/dvb/dvbX.demuxY/ we have:
	dev
	uevent
With the patch, we have (for a PCI DVB device):
	dev
	device -> ../../../devices/pci0000:00/0000:00:1e.0/0000:03:0d.0
	uevent
So userspace tools can (finally) work out which physical device a DVB adapter
refers to. Previously you had to kinda look through dmesg and hope that it
hadn't been dumped out of the buffer. This makes debugging a lot easier if
the system has been up for a long time!
This is done by adding an extra 'struct device *' parameter to
dvb_register_adapter(). It will work with any kind of standard
linux 'device'. Additionally, if someone has an embedded system which does
things differently, they can simply supply 'NULL' and the behaviour will be
as before - the link will simply not appear.

Ack'd-by: Manu Abraham <manu@linuxtv.org>
Acked-by: Michael Krufky <mkrufky@linuxtv.org>
Signed-off-by: Andrew de Quincey <adq_dvb@lidskialf.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>