device/pci: Declare pci_root_bus()

This is used a lot, cache the result so search
of domain from devicetree is only done once.

Improvement only applies when MAYBE_STATIC evaluates
to static.

Change-Id: If675abb632fe68acd59ba0bdfef854da3e0839a9
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/34004
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Nico Huber <nico.h@gmx.de>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

src/include/device/device.h

@@ -289,6 +289,7 @@ DEVTREE_CONST struct device *pcidev_path_behind(const struct bus *parent,
 		pci_devfn_t devfn);
 DEVTREE_CONST struct device *pcidev_path_on_root(pci_devfn_t devfn);
 DEVTREE_CONST struct device *pcidev_on_root(uint8_t dev, uint8_t fn);
+DEVTREE_CONST struct bus *pci_root_bus(void);
 
 void scan_smbus(struct device *bus);
 void scan_generic_bus(struct device *bus);