dt-bindings: mmc: describe new eMMC binding for fixed driver type

Some boards may have to use a certain driver type (or drive strength) to
achieve stable eMMC communication. Describe a binding to set this up via
DT.
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>

Documentation/devicetree/bindings/mmc/mmc.txt

@@ -53,6 +53,9 @@ Optional properties:
 - no-sdio: controller is limited to send sdio cmd during initialization
 - no-sd: controller is limited to send sd cmd during initialization
 - no-mmc: controller is limited to send mmc cmd during initialization
+- fixed-emmc-driver-type: for non-removable eMMC, enforce this driver type.
+  The value <n> is the driver type as specified in the eMMC specification
+  (table 206 in spec version 5.1).
 
 *NOTE* on CD and WP polarity. To use common for all SD/MMC host controllers line
 polarity properties, we have to fix the meaning of the "normal" and "inverted"