net: mvpp2: switch to SPDX identifiers

Use the appropriate SPDX license identifiers and drop the license text.
This patch is only cosmetic.

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