mips: hide conditionally unused functions

A couple of functions are defined unconditionally but have a conditional
declaration:

arch/mips/mm/tlb-r4k.c:461:12: error: no previous prototype for 'add_temporary_entry' [-Werror=missing-prototypes]
arch/mips/mm/pgtable-64.c:92:7: error: no previous prototype for 'mk_pmd' [-Werror=missing-prototypes]
arch/mips/mm/pgtable-64.c:101:6: error: no previous prototype for 'set_pmd_at' [-Werror=missing-prototypes]

Since there are no callers in these configurations, add the same #ifdef
checks around the definitions.

Link: https://lkml.kernel.org/r/20231204115710.2247097-19-arnd@kernel.org


Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Stephen Rothwell <sfr@rothwell.id.au>
Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>