[media] DocBook: Add a description for the Remote Controller interface

Adds a missing section to describe the remote controller interface.

The DocBook is just addin the same documentation as written at
Documentation/ABI/testing/sysfs-class-rc, using the DocBook's
way, and dropping timestamps/contact info.

While that means that we'll have the same info on two parts, there
are parts of the remote controller interface that doesn't belong
at Documentation/ABI/, and it makes sense to have everything
on the same place. This also means that we'll need to manually
track to be sure that both places will be synchronized, but, as
it is not expected much changes on it, this sync can be done
manually.

It also adds an introduction that states that the IR is a normal
evdev/input interface, plus the sysfs class nodes.

Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>