pci: Add general support for execution of video ROMs

Some platforms don't have native code for dealing with their video
hardware. In some cases they use a binary blob to set it up and perform
required actions like setting the video mode. This approach is a hangover
from the old PC days where a ROM was provided and executed during startup.

Even now, these ROMs are supplied as a way to set up video. It avoids the
code for every video chip needing to be provided in the boot loader. But
it makes the video much less flexible - e.g. it is not possible to do
anything else while the video init is happening (including waiting hundreds
of milliseconds for display panels to start up).

In any case, to deal with this sad state of affairs, provide an API for
execution of x86 video ROMs, either natively or through emulation.

Signed-off-by: Simon Glass <sjg@chromium.org>