drivers: of: Fix of_pci.h header guard

The compilation of of_pci.c is governed by CONFIG_OF_PCI, but the
corresponding declarations in of_pci.h are inconsistently guarded by
CONFIG_OF, with the result that if CONFIG_PCI is disabled for an OF
platform, the dangling external declarations are still active and the
inline stub definitions not. So far this has managed to go unnoticed
since it happens that the only references to these functions are from
code which itself depends on CONFIG_PCI or CONFIG_OF_PCI.

Fix this with the appropriate config guard so that any new callers
outside PCI-specific code don't start unexpectedly breaking under
certain configs.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Rob Herring <robh@kernel.org>