sandbox: Update PCI nodes in dts files

The way the PCI nodes are written today causes a number of warnings if
we stop disabling some of the warnings we pass to DTC.  As these
warnings aren't disabled in current Linux Kernel builds, we should aim
to not disable them here either, so rewrite these slightly.  Update the
driver model doc as well.

Cc: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Rini <trini@konsulko.com>