GPIO: MIPS: lantiq: convert gpio-stp-xway to OF

Implements OF support and add code to load custom properties from the DT.

The Serial To Parallel (STP) is found on MIPS based Lantiq socs. It is a
peripheral controller used to drive external shift register cascades. At most
3 groups of 8 bits can be driven. The hardware is able to allow the DSL modem
to drive the 2 LSBs of the cascade automatically. Newer socs are also able to
automatically drive some pins via the internal PHYs. The driver currently only
supports output functionality. Patches for the input feature found on newer
generations of the soc will be provided in a later series.
Signed-off-by: John Crispin <blogic@openwrt.org>
Cc: linux-kernel@vger.kernel.org
Cc: linux-mips@linux-mips.org
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Patchwork: https://patchwork.linux-mips.org/patch/3839/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

Documentation/devicetree/bindings/gpio/gpio-stp-xway.txt

+Lantiq SoC Serial To Parallel (STP) GPIO controller
+
+The Serial To Parallel (STP) is found on MIPS based Lantiq socs. It is a
+peripheral controller used to drive external shift register cascades. At most
+3 groups of 8 bits can be driven. The hardware is able to allow the DSL modem
+to drive the 2 LSBs of the cascade automatically.
+
+
+Required properties:
+- compatible : Should be "lantiq,gpio-stp-xway"
+- reg : Address and length of the register set for the device
+- #gpio-cells : Should be two.  The first cell is the pin number and
+  the second cell is used to specify optional parameters (currently
+  unused).
+- gpio-controller : Marks the device node as a gpio controller.
+
+Optional properties:
+- lantiq,shadow : The default value that we shall assume as already set on the
+  shift register cascade.
+- lantiq,groups : Set the 3 bit mask to select which of the 3 groups are enabled
+  in the shift register cascade.
+- lantiq,dsl : The dsl core can control the 2 LSBs of the gpio cascade. This 2 bit
+  property can enable this feature.
+- lantiq,phy1 : The gphy1 core can control 3 bits of the gpio cascade.
+- lantiq,phy2 : The gphy2 core can control 3 bits of the gpio cascade.
+- lantiq,rising : use rising instead of falling edge for the shift register
+
+Example:
+
+gpio1: stp@E100BB0 {
+	compatible = "lantiq,gpio-stp-xway";
+	reg = <0xE100BB0 0x40>;
+	#gpio-cells = <2>;
+	gpio-controller;
+
+	lantiq,shadow = <0xffff>;
+	lantiq,groups = <0x7>;
+	lantiq,dsl = <0x3>;
+	lantiq,phy1 = <0x7>;
+	lantiq,phy2 = <0x7>;
+	/* lantiq,rising; */
+};