drm/panel: Fix up DT bindings for Samsung lms397kf04

Improve the bindings and make them more usable:

- Pick in spi-cpha and spi-cpol from the SPI node parent,
  this will specify that we are "type 3" in the device tree
  rather than hardcoding it in the operating system.
- Drop the u32 ref from the SPI frequency: comes in from
  the SPI host bindings.
- Make spi-cpha, spi-cpol and port compulsory.
- Update the example with a real-world SPI controller,
  spi-gpio.

Cc: Noralf Trønnes <noralf@tronnes.org>
Cc: devicetree@vger.kernel.org
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210701213618.3818821-1-linus.walleij@linaro.org

Documentation/devicetree/bindings/display/panel/samsung,lms397kf04.yaml

@@ -33,8 +33,11 @@ properties:
 
   backlight: true
 
+  spi-cpha: true
+
+  spi-cpol: true
+
   spi-max-frequency:
-    $ref: /schemas/types.yaml#/definitions/uint32
     description: inherited as a SPI client node, the datasheet specifies
       maximum 300 ns minimum cycle which gives around 3 MHz max frequency
     maximum: 3000000
@@ -44,6 +47,9 @@ properties:
 required:
   - compatible
   - reg
+  - spi-cpha
+  - spi-cpol
+  - port
 
 additionalProperties: false
 
@@ -52,15 +58,23 @@ examples:
     #include <dt-bindings/gpio/gpio.h>
 
     spi {
+      compatible = "spi-gpio";
+      sck-gpios = <&gpio 0 GPIO_ACTIVE_HIGH>;
+      miso-gpios = <&gpio 1 GPIO_ACTIVE_HIGH>;
+      mosi-gpios = <&gpio 2 GPIO_ACTIVE_HIGH>;
+      cs-gpios = <&gpio 3 GPIO_ACTIVE_HIGH>;
+      num-chipselects = <1>;
       #address-cells = <1>;
       #size-cells = <0>;
       panel@0 {
         compatible = "samsung,lms397kf04";
         spi-max-frequency = <3000000>;
+        spi-cpha;
+        spi-cpol;
         reg = <0>;
         vci-supply = <&lcd_3v0_reg>;
         vccio-supply = <&lcd_1v8_reg>;
-        reset-gpios = <&gpio 1 GPIO_ACTIVE_LOW>;
+        reset-gpios = <&gpio 4 GPIO_ACTIVE_LOW>;
         backlight = <&ktd259>;
 
         port {