MIPS: page.h: Define virt_to_pfn()

Define virt_to_pfn() based on the existing definition of virt_to_page()
which already does a PFN_DOWN(vir_to_phys(kaddr)).

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Paul Burton <paul.burton@mips.com>
Cc: Maciej W. Rozycki <macro@linux-mips.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/15409/


Signed-off-by: James Hogan <jhogan@kernel.org>