arm: mvebu: armada-xp-theadorable.dts: Enable PCIe DT nodes

Now that the PCIe driver supports DM and DT parsing, enable the PCIe DT
nodes that are used by this board.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Dirk Eibach <dirk.eibach@gdsys.cc>
Cc: Mario Six <mario.six@gdsys.cc>
Cc: Chris Packham <chris.packham@alliedtelesis.co.nz>
Cc: Phil Sutter <phil@nwl.cc>
Cc: Marek Behún <marek.behun@nic.cz>
Cc: VlaoMao <vlaomao@gmail.com>