nios2: flush_tlb_all use TLBMISC way auto-increment feature

Writes to TLBACC cause TLBMISC way to be incremented, which can be
used to iterate over ways in a set, then wrap back to zero ready for
the next set. This reduces register writes significantly.

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