Introduce virt_to_phys()

virt_to_phys() returns the physical address given a virtual. In most
cases this will be just the input value as the vast majority of
systems run in a 1:1 mode.

However in systems that are not running this way it should report the
physical address or ~0 if no mapping exists for the given virtual
address.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>

include/asm-mips/io.h

@@ -118,7 +118,7 @@ static inline void set_io_port_base(unsigned long base)
  * Change virtual addresses to physical addresses and vv.
  * These are trivial on the 1:1 Linux/MIPS mapping
  */
-extern inline unsigned long virt_to_phys(volatile void * address)
+extern inline phys_addr_t virt_to_phys(void * address)
 {
 	return CPHYSADDR(address);
 }