dt-bindings: gpio: samsung: drop unused bindings

The commit 6c56c6cd ("gpio: samsung: Drop support for Exynos SoCs")
removed support for the Samsung Exynos SoC in lrgacy GPIO driver, since
it was moved to new pinctrl driver.  Remove old, unused bindings.
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>

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

-Samsung Exynos4 GPIO Controller
-
-Required properties:
-- compatible: Compatible property value should be "samsung,exynos4-gpio>".
-
-- reg: Physical base address of the controller and length of memory mapped
-  region.
-
-- #gpio-cells: Should be 4. The syntax of the gpio specifier used by client nodes
-  should be the following with values derived from the SoC user manual.
-     <[phandle of the gpio controller node]
-      [pin number within the gpio controller]
-      [mux function]
-      [flags and pull up/down]
-      [drive strength]>
-
-  Values for gpio specifier:
-  - Pin number: is a value between 0 to 7.
-  - Flags and Pull Up/Down: 0 - Pull Up/Down Disabled.
-                            1 - Pull Down Enabled.
-                            3 - Pull Up Enabled.
-          Bit 16 (0x00010000) - Input is active low.
-  - Drive Strength: 0 - 1x,
-                    1 - 3x,
-                    2 - 2x,
-                    3 - 4x
-
-- gpio-controller: Specifies that the node is a gpio controller.
-- #address-cells: should be 1.
-- #size-cells: should be 1.
-
-Example:
-
-	gpa0: gpio-controller@11400000 {
-		#address-cells = <1>;
-		#size-cells = <1>;
-		compatible = "samsung,exynos4-gpio";
-		reg = <0x11400000 0x20>;
-		#gpio-cells = <4>;
-		gpio-controller;
-	};