dt-bindings: mmc: Add information for DLL register properties

Add information regarding DLL register properties for getting board
specific configurations. These DLL register settings may vary from
board to board.

Signed-off-by: Sarthak Garg <sartgarg@codeaurora.org>
Link: https://lore.kernel.org/r/1590139950-7288-3-git-send-email-sartgarg@codeaurora.org


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