powerpc: Introduce ppc_pci_flags accessors

Currently there are a number of platforms that open code access to
the ppc_pci_flags global variable.  However, that variable is not
present if CONFIG_PCI is not set, which can lead to a build break.

This introduces a number of accessor functions that are defined
to be empty in the case of CONFIG_PCI being disabled.  The
various platform files in the kernel are updated to use these.

Signed-off-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>

arch/powerpc/platforms/44x/sam440ep.c

@@ -51,7 +51,7 @@ static int __init sam440ep_probe(void)
 	if (!of_flat_dt_is_compatible(root, "acube,sam440ep"))
 		return 0;
 
-	ppc_pci_flags = PPC_PCI_REASSIGN_ALL_RSRC;
+	ppc_pci_set_flags(PPC_PCI_REASSIGN_ALL_RSRC);
 
 	return 1;
 }