mmc: dt: Document binding for eMMC DDR 3.3V I/O voltage support

Cc: <devicetree@vger.kernel.org>
Cc: Rob Herring <robh@kernel.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Reviewed-by: Shawn Lin <shawn.lin@rock-chips.com>
Tested-by: Jan Glauber <jglauber@cavium.com>
Tested-by: Stefan Wahren <stefan.wahren@i2se.com>
Acked-by: Rob Herring <robh@kernel.org>