[MIPS] Fix PHYS_OFFSET for 64-bits kernels with 32-bits symbols

The current implementation of __pa() for 64-bits kernels with 32-bits
symbols is broken. In this configuration, we need 2 values for
PAGE_OFFSET, one in XKPHYS and the other in CKSEG0 space.

When the value in CKSEG0 space is used, it doesn't take into account
of PHYS_OFFSET. Even worse we can't redefine this value.

The patch restores CPHYSADDR() but in __pa()'s implementation because
it removes the need of 2 PAGE_OFFSET.

OTOH, CPHYSADDR() is quite bad when dealing with mapped kernels. So
this patch assumes there's no need to deal with such kernel in 64-bits
world.

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