dt-bindings: gpio: aspeed: Add SGPIO support

Add bindings to support SGPIO on AST2400 or AST2500.
Signed-off-by: Hongwei Zhang <hongweiz@ami.com>
Reviewed-by: Andrew Jeffery <andrew@aj.id.au>
Link: https://lore.kernel.org/r/1564603297-1391-2-git-send-email-hongweiz@ami.com
[Adjusted when applying in several ways]
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

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

+Aspeed SGPIO controller Device Tree Bindings
+--------------------------------------------
+
+This SGPIO controller is for ASPEED AST2500 SoC, it supports up to 80 full
+featured Serial GPIOs. Each of the Serial GPIO pins can be programmed to
+support the following options:
+- Support interrupt option for each input port and various interrupt
+  sensitivity option (level-high, level-low, edge-high, edge-low)
+- Support reset tolerance option for each output port
+- Directly connected to APB bus and its shift clock is from APB bus clock
+  divided by a programmable value.
+- Co-work with external signal-chained TTL components (74LV165/74LV595)
+
+Required properties:
+
+- compatible : Should be one of
+  "aspeed,ast2400-sgpio", "aspeed,ast2500-sgpio"
+- #gpio-cells : Should be 2, see gpio.txt
+- reg : Address and length of the register set for the device
+- gpio-controller : Marks the device node as a GPIO controller
+- interrupts : Interrupt specifier, see interrupt-controller/interrupts.txt
+- interrupt-controller : Mark the GPIO controller as an interrupt-controller
+- ngpios : number of GPIO lines, see gpio.txt
+  (should be multiple of 8, up to 80 pins)
+- clocks : A phandle to the APB clock for SGPM clock division
+- bus-frequency : SGPM CLK frequency
+
+The sgpio and interrupt properties are further described in their respective
+bindings documentation:
+
+- Documentation/devicetree/bindings/gpio/gpio.txt
+- Documentation/devicetree/bindings/interrupt-controller/interrupts.txt
+
+  Example:
+	sgpio: sgpio@1e780200 {
+		#gpio-cells = <2>;
+		compatible = "aspeed,ast2500-sgpio";
+		gpio-controller;
+		interrupts = <40>;
+		reg = <0x1e780200 0x0100>;
+		clocks = <&syscon ASPEED_CLK_APB>;
+		interrupt-controller;
+		ngpios = <8>;
+		bus-frequency = <12000000>;
+	};