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.com


Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>