msm: mdss: Update binding Document for mdss_mdp3

In mdss dtsi compatible property can be either mdss_mdp or
mdss_mdp3, based on mdp5 or mdp3 target. Update the binding
document to capture the same.

Change-Id: If66f65273c2246dc544e58abd0217b42c9714ea1
Signed-off-by: Shivaraj Shetty <shivaraj@codeaurora.org>