media: pci: move VIDEO_PCI_SKELETON to a different Kconfig

The V4L2 PCI skeleton is not part of the V4L2 core. Move it
to appear together with the other PCI drivers, at the end,
as this is something that normal users don't even need to
bother.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>