net: mvpp2: make the per-cpu helpers static

The Marvell PPv2 driver has per-cpu functions. As they only are used in
the main file, make them static and remove their prototype from the
header.

Signed-off-by: Antoine Tenart <antoine.tenart@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>