MIPS: Make phys_to_virt utilize __va()

The implementation is exactly the same, so avoid open-coding it in two
different locations.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Serge Semin <fancer.lancer@gmail.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>