dt-bindings: new binding for Himax HX8357D display panels

This adds a new binding for Himax HX8357D display panels. It includes
a compatible string for one display (more can be added in the future).

The YX350HV15 panel[1] is found in the Adafruit PiTFT 3.5" Touch
Screen for Raspberry Pi.

[1] https://learn.adafruit.com/adafruit-pitft-3-dot-5-touch-screen-for-raspberry-pi/downloads

This binding is closely modeled after the ili9341 binding, for a
similar product from adafruit.  The primary difference is that the
hx8357d doesn't have a reset line that I can find in the schematics.

v2: Document the "reg" property (requested by Rob), fix commit message
    typo (Noralf)
Signed-off-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20181024184313.2967-2-eric@anholt.netReviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Noralf Trønnes <noralf@tronnes.org>

Documentation/devicetree/bindings/display/himax,hx8357d.txt

+Himax HX8357D display panels
+
+This binding is for display panels using a Himax HX8357D controller in SPI
+mode, such as the Adafruit 3.5" TFT for Raspberry Pi.
+
+Required properties:
+- compatible:	"adafruit,yx350hv15", "himax,hx8357d"
+- dc-gpios:	D/C pin
+- reg:		address of the panel on the SPI bus
+
+The node for this driver must be a child node of a SPI controller, hence
+all mandatory properties described in ../spi/spi-bus.txt must be specified.
+
+Optional properties:
+- rotation:	panel rotation in degrees counter clockwise (0,90,180,270)
+- backlight:	phandle of the backlight device attached to the panel
+
+Example:
+	display@0{
+		compatible = "adafruit,yx350hv15", "himax,hx8357d";
+		reg = <0>;
+		spi-max-frequency = <32000000>;
+		dc-gpios = <&gpio0 25 GPIO_ACTIVE_HIGH>;
+		rotation = <90>;
+		backlight = <&backlight>;
+	};