[media] tvp5150: add HW input connectors support

The tvp5150 decoder has different input connectors. The actual list of
HW inputs depends on the device version but all have at least these 3:

1) Composite0
2) Composite1
3) S-Video

and some variants have a 4th possible input connector:

4) Signal generator

The driver currently uses the .s_routing callback to switch the input
connector but since these are separate HW blocks, it's better to use
media entities to represent the input connectors and their source pads
linked with the decoder's sink pad.

This allows user-space to use the MEDIA_IOC_SETUP_LINK ioctl to choose
the input connector. For example using the media-ctl user-space tool:

$ media-ctl -r -l '"Composite0":0->"tvp5150 1-005c":0[1]'

Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>