[MIPS] Make free_init_pages() arguments to be physical addresses

It allows caller of this function to not care about CKSEG0/XKPHYS
address mixes. It's now automatically done by free_init_pages().

We can now safely remove hack needed by 64 bit kernels with
CONFIG_BUILD_ELF64=n in free_initmem().

Signed-off-by: Franck Bui-Huu <fbuihuu@gmail.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>