gpio: aspeed: Update documentation with ast2600 controllers

The ast2600 is a new generation of SoC from ASPEED. Similarly to the
ast2400 and ast2500, it has a GPIO controller for it's 3.3V GPIO pins.
Additionally, it has a GPIO controller for 36 1.8V GPIO pins.  We use
the ngpio property to differentiate between these controllers.
Signed-off-by: Rashmica Gupta <rashmica.g@gmail.com>
Link: https://lore.kernel.org/r/20190906062547.13264-1-rashmica.g@gmail.comSigned-off-by: Linus Walleij <linus.walleij@linaro.org>

Documentation/devicetree/bindings/gpio/gpio-aspeed.txt

@@ -2,7 +2,8 @@ Aspeed GPIO controller Device Tree Bindings
 -------------------------------------------
 
 Required properties:
-- compatible		: Either "aspeed,ast2400-gpio" or "aspeed,ast2500-gpio"
+- compatible		: Either "aspeed,ast2400-gpio", "aspeed,ast2500-gpio",
+					or "aspeed,ast2600-gpio".
 
 - #gpio-cells 		: Should be two
 			  - First cell is the GPIO line number
@@ -17,7 +18,9 @@ Required properties:
 
 Optional properties:
 
-- clocks                : A phandle to the clock to use for debounce timings
+- clocks		: A phandle to the clock to use for debounce timings
+- ngpios		: Number of GPIOs controlled by this controller. Should	be set
+				  when there are multiple GPIO controllers on a SoC (ast2600).
 
 The gpio and interrupt properties are further described in their respective
 bindings documentation: