[MIPS] setup.c: get ride of CPHYSADDR()

and use new __pa() implementation instead introduced by the previous
patch. Indeed this macro can be used now even by the 64 bit kernels
with CONFIG_BUILD_ELF64=n config.

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