mmc: dw_mmc: Add "disable-wp" device tree property

The "disable-wp" property is used to specify that a given SD card slot
doesn't have a concept of write protect.  This eliminates the need for
special case code for SD slots that should never be write protected
(like a micro SD slot or a dev board).

The dw_mmc driver is special in needing to specify "disable-wp"
because the lack of a "wp-gpios" property means to use the special
purpose write protect line.  On some other mmc devices the lack of
"wp-gpios" means that write protect should be disabled.

Signed-off-by: Doug Anderson <dianders@chromium.org>
Acked-by: Seungwon Jeon <tgih.jun@samsung.com>
Acked-by: Will Newton <will.newton@imgtec.com>
Acked-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Chris Ball <cjb@laptop.org>

Documentation/devicetree/bindings/mmc/synopsis-dw-mshc.txt

@@ -26,8 +26,16 @@ Required Properties:
 	* bus-width: as documented in mmc core bindings.
 
 	* wp-gpios: specifies the write protect gpio line. The format of the
-	  gpio specifier depends on the gpio controller. If the write-protect
-	  line is not available, this property is optional.
+	  gpio specifier depends on the gpio controller. If a GPIO is not used
+	  for write-protect, this property is optional.
+
+	* disable-wp: If the wp-gpios property isn't present then (by default)
+	  we'd assume that the write protect is hooked up directly to the
+	  controller's special purpose write protect line (accessible via
+	  the WRTPRT register).  However, it's possible that we simply don't
+	  want write protect.  In that case specify 'disable-wp'.
+	  NOTE: This property is not required for slots known to always
+	  connect to eMMC or SDIO cards.
 
 Optional properties: