dt-bindings: mmc: sdhci-am654: Add ti,clkbuf-sel binding

Add documentation for the clock buffer select phy property in the
am654x and j721e 4 bit IP
Signed-off-by: Faiz Abbas <faiz_abbas@ti.com>
Link: https://lore.kernel.org/r/20200619125801.9530-2-faiz_abbas@ti.comSigned-off-by: Ulf Hansson <ulf.hansson@linaro.org>

Documentation/devicetree/bindings/mmc/sdhci-am654.txt

@@ -39,6 +39,7 @@ Optional Properties (Required for ti,am654-sdhci-5.1 and ti,j721e-sdhci-8bit):
 				  Valid values are 33, 40, 50, 66 and 100 ohms.
 Optional Properties:
 	- ti,strobe-sel: strobe select delay for HS400 speed mode. Default value: 0x0.
+	- ti,clkbuf-sel: Clock Delay Buffer Select
 
 Example: