[MIPS] Make __pa() aware of XKPHYS/CKSEG0 address mix for 64 bit kernels

During early boot mem init, some configs couldn't use __pa() to
convert virtual into physical addresses. Specially for 64 bit
kernel cases when CONFIG_BUILD_ELF64=n. This patch make __pa()
work for _all_ configs and thus make CPHYSADDR() useless.

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