[coreboot] More device cleanup

[Myles Watson]

This patch continues the device code cleanup.

The largest changes are to get_pci_bridge_ops, and related changes to make
it
compile and use correct declarations.  I'd appreciate someone else checking
my const usage for sanity.

While I was doing that I moved the checks for CONFIG_<BUS>_PLUGIN_SUPPORT to
the Makefile.

The only functional difference is a possible NULL dereference in a debug
statement.

Signed-off-by: Myles Watson

Thanks,
Myles