ppc4xx: Consolidate pci_pre_init() function

This patch removes the duplicted implementations of the pci_pre_init()
function by introducing a weak default function for it. This weak default
has a different implementation for some PPC variants. It can be
overridden by a board specific version.

Signed-off-by: Stefan Roese <sr@denx.de>
diff --git a/cpu/ppc4xx/cpu.c b/cpu/ppc4xx/cpu.c
index e1b00a7..73d4d06 100644
--- a/cpu/ppc4xx/cpu.c
+++ b/cpu/ppc4xx/cpu.c
@@ -81,7 +81,7 @@
 
 #if defined(CONFIG_PCI) && !defined(CONFIG_IOP480) && \
     !defined(CONFIG_405) && !defined(CONFIG_405EX)
-static int pci_arbiter_enabled(void)
+int pci_arbiter_enabled(void)
 {
 #if defined(CONFIG_405GP)
 	return (mfdcr(CPC0_PSR) & PSR_PCI_ARBIT_EN);