nios2: change virt_to_phys to use physaddr_mask in global data

As virt_to_phys() is used a lot in DMA transfer, change it
to use physaddr_mask in global data. This will save an "if"
statement and get a little faster.

Signed-off-by: Thomas Chou <thomas@wytron.com.tw>
Acked-by: Marek Vasut <marex@denx.de>