Platform/RaspberryPi: Add PCIe SSDT

Since we plan on toggling between XHCI and PCI the PCI
root needs to be in its own SSDT. This is all thats needed
of UEFI. The SMC conduit is provided directly to the running
OS. When the OS detects this PCIe port on a machine without
a MCFG it attempts to connect to the SMC conduit. The RPi
definition doesn't have any power mgmt, and only provides
a description of the root port.

Signed-off-by: Jeremy Linton <jeremy.linton@arm.com>
Tested-by: Jared McNeill <jmcneill@invisible.ca>