drm/msm/hdmi: Clean up HDMI gpio DT bindings

Make the following changes in the HDMI gpio bindings:

- Use "-gpios" as the suffix for all the gpio names
- Move all the gpios to optional, since there are platforms that use none
  of them.
- The HPD gpio is a standard one, remove the "qcom,hdmi-tx-" prefix from
  it.
- Remove the HDMI DDC clk/data gpios. They are just leftovers of an old
  way to configure pinctrl properties.
- Add a missing lpm gpio used on some platforms.

Make the necessary changes in the driver to incorporate these changes.

There hasn't been any upstream DT that uses the HDMI bindings, so it's
okay to change and move around these properties.

Cc: Rob Herring <robh@kernel.org>
Cc: devicetree@vger.kernel.org
Signed-off-by: Archit Taneja <architt@codeaurora.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>