Document the bindings used for the sn65dsi86 DSI to eDP bridge.

Changes in v1:
 - Rephrase the dt-binding descriptions to be more inline with existing
   bindings (Andrzej Hajda).
 - Add missing dt-binding that are parsed by corresponding driver
   (Andrzej Hajda).

Changes in v2:
 - Remove edp panel specific dt-binding entries. Only keep bridge
   specific entries (Sean Paul).
 - Remove custom-modes dt entry since its usage is removed from driver also (Sean Paul).
 - Remove is-pluggable dt entry since this will not be needed anymore (Sean Paul).

Changes in v3:
 - Remove irq-gpio dt entry and instead populate is an interrupt
   property (Rob Herring).

Changes in v4:
 - Add link to bridge chip datasheet (Stephen Boyd)
 - Add vpll and vcc regulator supply bindings (Stephen Boyd)
 - Add ref clk optional dt binding (Stephen Boyd)
 - Add gpio-controller optional dt binding (Stephen Boyd)

Changes in v5:
 - Use clock property to specify the input refclk (Stephen Boyd).
 - Update gpio cell and pwm cell numbers (Stephen Boyd).

Changes in v6:
 - Add property to mention the lane mapping scheme and polarity inversion
   (Stephen Boyd).

Changes in v7:
 - Detail description of lane mapping scheme dt property (Andrzej
   Hajda/ Rob Herring).
 - Removed HDP gpio binding, since the bridge uses IRQ signal to
   determine HPD, and IRQ property is already documented in binding.

Changes in v8:
 - Removed unnecessary explanation of lane mapping and polarity dt
   property, since these are already explained in media/video-interface
   dt binidng (Rob Herring).

Changes in v9:
 - Avoid putting re-definition of lane mapping and polarity dt binding
   (Rob Herring).

Changes in v10:
 - Use interrupts-extended property instead of interrupts to specify
   interrupt line (Andrzej Hajda).
 - Move data-lanes and lane-polarity property example to proper place (Andrzej Hajda).

Changes in v11:
 - Add a property for suspend gpio function of GPIO1 pin on bridge chip
   (Stephen Boyd).

Changes in v12:
 - Remove binding for dedicated DDC line (Andrzej Hajda).
Signed-off-by: Sandeep Panda <spanda@codeaurora.org>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180716084330.26698-3-spanda@codeaurora.org