dt-binding: mtd: physmap: Document the addr-gpios property

Add documentation for the addr-gpios. This extension to the physmap
binding allow creating flash devices that are paged using GPIOs.

Cc: devicetree@vger.kernel.org
Signed-off-by: Ricardo Ribalda Delgado <ricardo.ribalda@gmail.com>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Linus Walleij <linus.walleij@linaro.org>

Documentation/devicetree/bindings/mtd/mtd-physmap.txt

@@ -29,6 +29,8 @@ file systems on embedded devices.
  - use-advanced-sector-protection: boolean to enable support for the
    advanced sector protection (Spansion: PPB - Persistent Protection
    Bits) locking.
+ - addr-gpios : (optional) List of GPIO descriptors that will be used to
+   address the MSBs address lines. The order goes from LSB to MSB.
 
 For JEDEC compatible devices, the following additional properties
 are defined: