[MIPS] Introduce __pa_symbol()

This patch introduces __pa_symbol() macro which should be used to
calculate the physical address of kernel symbols. It also relies
on RELOC_HIDE() to avoid any compiler's oddities when doing
arithmetics on symbols.

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