MIPS: Octeon: Opt-out 4k_cache feature

Octeon has a different cache interface with traditional R4K one,
just opt-out this flag for octeon to avoid run R4K cache initialization
code accidentally.

Also remove ISA level assumption for 4k cache.

Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>