ARM: dts: BCM5301X: Specify pcie2 in the DT

The SoC supports three pcie ports. Currently, only
pcie0 and pcie1 are enabled. This patch adds the
pcie2 port as well.

Signed-off-by: Christian Lamparter <chunkeey@gmail.com>
Reviewed-by: Scott Branden <scott.branden@broadcom.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>