dt-bindings: msi: add Synopsys devicetree entry

To use Synopsys MSI, users must add qcom,snps to MSI
devicetree node, otherwise QGIC MSI will be supported.

Change-Id: I5aed9412387c0eb5d9c56bc739fedf156a14f958
Signed-off-by: Tony Truong <truong@codeaurora.org>