arm64: Juno: Add support for the PCIe host bridge on Juno R1

arm64: Juno: Add support for the PCIe host bridge on Juno R1

Juno R1 board sports a functional PCIe host bridge that is
compliant with the SBSA standard found [1] here. With the right
firmware that initialises the XpressRICH3 controller one can
use the generic Host Bridge driver to use the PCIe hardware.

Signed-off-by: Liviu Dudau <Liviu.Dudau@arm.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
[1] http://infocenter.arm.com/help/topic/com.arm.doc.den0029a/