lib/powerpc: addrmap_phys_to_virt() should return a pointer

lib/powerpc: addrmap_phys_to_virt() should return a pointer

addrmap_phys_to_virt() converts a physical address (phys_addr_t) to a
virtual address, so it should return a pointer instead of an unsigned long.
Its counterpart, addrmap_virt_to_phys(), takes a pointer, so now they're
orthogonal.

The only caller of addrmap_phys_to_virt() converts the return value to
a pointer anyway.

Signed-off-by: Timur Tabi <timur@freescale.com>
Signed-off-by: Andy Fleming <afleming@freescale.com>