of: implement pci_address_to_pio as weak function

Implement pci_address_to_pio as weak function to remove the dependency on
asm/prom.h. This is in preparation to make prom.h optional.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Acked-by: Grant Likely <grant.likely@linaro.org>
Cc: Michal Simek <monstr@monstr.eu>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: x86@kernel.org
Cc: Grant Likely <grant.likely@linaro.org>
diff --git a/include/linux/of_address.h b/include/linux/of_address.h
index 4c2e6f2..f6fc689 100644
--- a/include/linux/of_address.h
+++ b/include/linux/of_address.h
@@ -52,10 +52,7 @@
 extern const __be32 *of_get_address(struct device_node *dev, int index,
 			   u64 *size, unsigned int *flags);
 
-#ifndef pci_address_to_pio
-static inline unsigned long pci_address_to_pio(phys_addr_t addr) { return -1; }
-#define pci_address_to_pio pci_address_to_pio
-#endif
+extern unsigned long pci_address_to_pio(phys_addr_t addr);
 
 extern int of_pci_range_parser_init(struct of_pci_range_parser *parser,
 			struct device_node *node);