net: mvpp2: fix include guards in mvpp2_prs.h

Include guards should be put before #includes. This doesn't fix any bug,
but prevent future compilation issues when adding new files in the mvpp2
driver

The Header Parser init function needs the platform_device definition,
and with the fixed include guards we need to add the missing include.

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