ARM: dts: msm: add pci DT nodes for PCIe switch for sdxprairie

If there is a switch connected to PCIe root complex, the
devicetree heirarchy needs to correctly reflect the physical
connection. Add pci DT nodes for each switch device such as
switch upstream and downstream ports.

Change-Id: Ic169c6bc3387c0adc0259dfa23c8c55438169045
Signed-off-by: Tony Truong <truong@codeaurora.org>