nios2: fix virt_to_phys for nios2 with MMU

As the virtual address and physical address mapping of nios2 with
MMU are different. Add a check of MMU, and fix the mapping.

Signed-off-by: Thomas Chou <thomas@wytron.com.tw>
Acked-by: Marek Vasut <marex@denx.de>
Reviewed-by: Ley Foon Tan <lftan@altera.com>