Documentation/bindings: net: marvell-pp2: add the system controller

This patch documents the new marvell,system-controller property used by
the Marvell ppv2 network driver.

Signed-off-by: Antoine Tenart <antoine.tenart@free-electrons.com>
Tested-by: Marcin Wojtas <mw@semihalf.com>
Signed-off-by: David S. Miller <davem@davemloft.net>