Add the compatible string for 14nm DSI PHY (used in MSM8996/APQ8096).
>From 14nm PHY onwards, the "dsi_phy_regulator" reg-name is not required,
but "dsi_phy_lane" reg-name is. Update the doc to specify the reg-names
each PHY revision needs.

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