dt-bindings: sdhci-omap: Add note for cpu_thermal

The driver fetches a thermal zone using the string "cpu_thermal" for
tuning operation. Add a note for the same.
Signed-off-by: Faiz Abbas <faiz_abbas@ti.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>

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

@@ -2,6 +2,8 @@
 
 Refer to mmc.txt for standard MMC bindings.
 
+For UHS devices which require tuning, the device tree should have a "cpu_thermal" node which maps to the appropriate thermal zone. This is used to get the temperature of the zone during tuning.
+
 Required properties:
 - compatible: Should be "ti,dra7-sdhci" for DRA7 and DRA72 controllers
 	      Should be "ti,k2g-sdhci" for K2G