nios2: update_mmu_cache preload the TLB with the new PTE

Rather than flush the TLB entry when installing a new PTE to allow
the fast TLB reload to re-fill the TLB, just refill the TLB entry
when removing the old one.

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