ARC: mm: tlb flush optim: Make TLBWriteNI fallback to TLBWrite if not available

TLBWriteNI was introduced in MMUv2 (to not invalidate uTLBs in Fast Path
TLB Refill Handler). To avoid #ifdef'ery make it fallback to TLBWrite availabel on all MMUs. This will also help with next change

Signed-off-by: Vineet Gupta <vgupta@synopsys.com>