dt-bindings: mmc: sdhci-msm: correct minimum number of clocks

In the TXT binding before conversion, the "xo" clock was listed as
optional.  Conversion kept it optional in "clock-names", but not in
"clocks".  This fixes dbts_check warnings like:

  qcom-sdx65-mtp.dtb: mmc@8804000: clocks: [[13, 59], [13, 58]] is too short

Cc: <stable@vger.kernel.org>
Fixes: a4553772

 ("dt-bindings: mmc: sdhci-msm: Convert bindings to yaml")
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20230825135503.282135-1-krzysztof.kozlowski@linaro.org
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>