dt-bindings: mmc: sdhci-am654: Document bindings for the host controllers on TI's J721E devices.

Add binding documentation for mmc host controllers present on TI's J721E
SOC. The 4 bit IP on J721E doesn't have a phy DLL so make DLL related
properties as optional for that compatible. Also add an optional
strobe-sel property used for HS400 speed mode.

Signed-off-by: Faiz Abbas <faiz_abbas@ti.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>