net: mvpp2: make mvpp2_read_relaxed static

In the Marvell PPv2 driver the mvpp2_read_relaxed function is only used
in a single file. Make it static and remove its prototype from the
header.

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