No point in checking cpu_has_tlb before we've computed the CPU options.

So for now we just unconditionally set the option - Linux wouldn't
work without a TLB anyway.
    
Setting MIPS_CPU_4KTLB was missing for Alchemy and Sandcraft, add that
back.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>