mips: move cache declarations into header

Some of the cache functions are declared only for their callers, e.g.

arch/mips/mm/c-r3k.c:28:15: error: no previous prototype for 'r3k_cache_size' [-Werror=missing-prototypes]
arch/mips/mm/c-r3k.c:63:15: error: no previous prototype for 'r3k_cache_lsize' [-Werror=missing-prototypes]
arch/mips/mm/c-r4k.c:1703:6: error: no previous prototype for 'r4k_cache_init' [-Werror=missing-prototypes]
arch/mips/mm/sc-mips.c:255:5: error: no previous prototype for 'mips_sc_init' [-Werror=missing-prototypes]

Move all the declarations to asm/cache.h and asm/r4kcache.h where they
can be seen by the function definitions.

Link: https://lkml.kernel.org/r/20231204115710.2247097-13-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>