powerpc/5xxx: Add common mpc5xxx_get_bus_frequency() function

So far, MPC512x used mpc512x_find_ips_freq() to get the bus frequency,
while MPC52xx used mpc52xx_find_ipb_freq().  Despite the different
clock names (IPS vs. IPB) the code was identical.

Use common code for both processor families.

Signed-off-by: Wolfgang Denk <wd@denx.de>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
diff --git a/drivers/watchdog/mpc5200_wdt.c b/drivers/watchdog/mpc5200_wdt.c
index 465fe36..fa9c47c 100644
--- a/drivers/watchdog/mpc5200_wdt.c
+++ b/drivers/watchdog/mpc5200_wdt.c
@@ -188,7 +188,7 @@
 	if (!wdt)
 		return -ENOMEM;
 
-	wdt->ipb_freq = mpc52xx_find_ipb_freq(op->node);
+	wdt->ipb_freq = mpc5xxx_get_bus_frequency(op->node);
 
 	err = of_address_to_resource(op->node, 0, &wdt->mem);
 	if (err)