nios2: flush_tlb_mm flush only the pid

Currently flush_tlb_mm flushes the entire TLB. Switch it to doing a
PID aware flush. This also improves the readibility of flush_tlb_pid.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Ley Foon Tan <ley.foon.tan@intel.com>