MIPS: Export tlbmiss_handler_setup_pgd near its definition

We export tlbmiss_handler_setup_pgd in arch/mips/mm/tlbex.c close to a
declaration of it, rather than close to its definition as is standard.

We've supported exporting symbols in assembly code since commit
22823ab4 ("EXPORT_SYMBOL() for asm"), so move the export to follow
the function's (stub) definition.

Signed-off-by: Paul Burton <paul.burton@mips.com>