ppc4xx: Remove duplicated is_pci_host() functions

This patch introduces a weak default function for is_pci_host(),
returning 1. This is the default behaviour, since most boards only
implement PCI host functionality. This weak default can be overridden
by a board specific version if needed.

Signed-off-by: Stefan Roese <sr@denx.de>
diff --git a/board/xes/xpedite1000/xpedite1000.c b/board/xes/xpedite1000/xpedite1000.c
index 04949bb..0c4cf94 100644
--- a/board/xes/xpedite1000/xpedite1000.c
+++ b/board/xes/xpedite1000/xpedite1000.c
@@ -200,6 +200,8 @@
 
 #if defined(CONFIG_PCI)
 /*
+ * Override weak is_pci_host()
+ *
  * This routine is called to determine if a pci scan should be
  * performed. With various hardware environments (especially cPCI and
  * PPMC) it's insufficient to depend on the state of the arbiter enable