microblaze: mm: Flush TLB to ensure correct mapping when higmem ON

MMU contains invalid mapping which wasn't flushed and new mapping
is using the same addresses as previous one. That's why TLB miss is not
happening to get new correct TLB entry and MMU points to incorrect area.

This is replicatable when large files(256MB and more) are copied and
checked.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>