powerpc/mpc5xxx: fix sparse warning for non static symbol

Fix warning:
symbol 'mpc5xxx_get_bus_frequency' was not declared. Should it be static?
Signed-off-by: Anatolij Gustschin <agust@denx.de>

arch/powerpc/sysdev/mpc5xxx_clocks.c

@@ -9,9 +9,9 @@
 #include <linux/kernel.h>
 #include <linux/of_platform.h>
 #include <linux/export.h>
+#include <asm/mpc5xxx.h>
 
-unsigned int
-mpc5xxx_get_bus_frequency(struct device_node *node)
+unsigned long mpc5xxx_get_bus_frequency(struct device_node *node)
 {
 	struct device_node *np;
 	const unsigned int *p_bus_freq = NULL;