dt-bindings: mmc: sdhci-am654: Document bindings for the host controllers on TI's J7200 devices

Add binding documentation for mmc host controllers present on
TI's J7200 SOC

Signed-off-by: Faiz Abbas <faiz_abbas@ti.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20200802070114.9624-1-faiz_abbas@ti.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>