dt-bindings: display: Add Sitronix ST7701 panel documentation

Techstar TS8550B MIPI DSI panel is 480x854, 2-lane MIPI DSI LCD panel
with inbuilt ST7701 chip.

The default regulator names in ST7701 chip is renamed in Techstar TS8550B
so, add specific binding names for them.
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190124215131.17452-1-jagan@amarulasolutions.com

Documentation/devicetree/bindings/display/panel/sitronix,st7701.txt

+Sitronix ST7701 based LCD panels
+
+ST7701 designed for small and medium sizes of TFT LCD display, is
+capable of supporting up to 480RGBX864 in resolution. It provides
+several system interfaces like MIPI/RGB/SPI.
+
+Techstar TS8550B is 480x854, 2-lane MIPI DSI LCD panel which has
+inbuilt ST7701 chip.
+
+Required properties:
+- compatible: must be "sitronix,st7701" and one of
+  * "techstar,ts8550b"
+- reset-gpios: a GPIO phandle for the reset pin
+
+Required properties for techstar,ts8550b:
+- reg: DSI virtual channel used by that screen
+- VCC-supply: analog regulator for MIPI circuit
+- IOVCC-supply: I/O system regulator
+
+Optional properties:
+- backlight: phandle for the backlight control.
+
+panel@0 {
+	compatible = "techstar,ts8550b", "sitronix,st7701";
+	reg = <0>;
+	VCC-supply = <&reg_dldo2>;
+	IOVCC-supply = <&reg_dldo2>;
+	reset-gpios = <&pio 3 24 GPIO_ACTIVE_HIGH>; /* LCD-RST: PD24 */
+	backlight = <&backlight>;
+};