net: mvpp2: rename mvpp2_percpu function to mvpp2_thread

As the mvpp2_percpu_read/write/... functions aren't really per-cpu but
per s/w thread, rename them to include 'thread' instead of 'percpu'.
This is a cosmetic patch.
Signed-off-by: Antoine Tenart <antoine.tenart@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>