mips: mark local function static if possible

These two functions are global but have no extern prototypes or other
callers, so it's best to mark them as static, avoiding these warnings:

arch/mips/kernel/mips-cm.c:204:13: error: no previous prototype for '__mips_cm_l2sync_phys_base' [-Werror=missing-prototypes]
arch/mips/mm/c-r4k.c:1827:12: error: no previous prototype for 'r4k_cache_init_pm' [-Werror=missing-prototypes]

Link: https://lkml.kernel.org/r/20231204115710.2247097-7-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>