dt-bindings: sdhci-msm: Add xo value

Add "xo" value which is tcxo clock to sdhci-msm
DT binding properties.
Signed-off-by: Ritesh Harjani <riteshh@codeaurora.org>
Reviewed-by: Stephen Boyd <sboyd@codeaurora.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>

Documentation/devicetree/bindings/mmc/sdhci-msm.txt

@@ -17,6 +17,7 @@ Required properties:
 	"iface" - Main peripheral bus clock (PCLK/HCLK - AHB Bus clock) (required)
 	"core"	- SDC MMC clock (MCLK) (required)
 	"bus"	- SDCC bus voter clock (optional)
+	"xo"	- TCXO clock (optional)
 
 Example: