nios2: Use an invalid TLB entry address helper function

There is no need for complicated calculation for an invalid address
that maps to the same TLB index as the entry to be invalidated. Using
the TLB address plus the two top bits set puts the address into the
kernel TLB bypass range and still maps to the same cache line.

This is also a bug fix for flush_tlb_pid, which is currently unused,
but does not set PTEADDR to invalid.

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