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>