drm/bridge: Add bindings for TI THS8134

This adds device tree bindings for the Texas Instruments
THS8134, THS8134A and THS8134B VGA DACs by extending and
renaming the existing bindings for THS8135.

These DACs are used for the VGA outputs on the ARM reference
designs such as Integrator, Versatile and RealView.

Cc: devicetree@vger.kernel.org
Cc: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Archit Taneja <architt@codeaurora.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20180112074854.9560-1-linus.walleij@linaro.org