Add a PCI Express root complex capability block so the PCI layer identifies
the bridge as a PCI Express device.

We expose this as a version 1 PCIe capability block, with slot support.  We
disable the clock power management capability as this depends on boards
wiring the CLKREQ# signal.

Tested-by: Willy Tarreau <w@1wt.eu> (Iomega iConnect Kirkwood, MiraBox Armada 370)
Tested-by: Andrew Lunn <andrew@lunn.ch> (D-Link DIR664 Kirkwood)
Tested-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com> (Armada XP GP)
Signed-off-by: NRussell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: NBjorn Helgaas <bhelgaas@google.com>
Reviewed-by: NThomas Petazzoni <thomas.petazzoni@free-electrons.com>