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