mips: cavium-octeon: Move octeon-crypto.c into parent dir

Since octeon-crypto.c is the only remaining source file in
arch/mips/cavium-octeon/crypto/, move it into its parent directory
arch/mips/cavium-octeon/.  Then remove the directory
arch/mips/cavium-octeon/crypto/, including its Makefile.

Link: https://lore.kernel.org/r/20250805222855.10362-4-ebiggers@kernel.org


Signed-off-by: Eric Biggers <ebiggers@kernel.org>