dt-bindings: marvell,prestera: Add description for device-tree bindings

Add brief description how to configure base mac address binding in
device-tree.

Describe requirement for the PCI port which is connected to the ASIC, to
allow access to the firmware related registers.

Signed-off-by: Vadym Kochan <vadym.kochan@plvision.eu>
Signed-off-by: David S. Miller <davem@davemloft.net>