ARM: dts: imx6q-bx50v3: Add internal switch

B850v3, B650v3 and B450v3 all have a GPIO bit banged MDIO bus to
communicate with a Marvell switch. On all devices the switch is
connected to a PCI based network card, which needs to be referenced
by DT, so this also adds the common PCI root node.

Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>