mdss: mhl3: rename "sil,i2c_port#" DT property to "sil,i2c-port"

Update i2c port number property name for SiI 8620 driver
to use dash instead of underscore to align with proper
dt guidelines. Also document i2c port number property
as an optional property.

Change-Id: I471d22eb2d0de9ffd5a43ece764dffd97e8ed6a8
Signed-off-by: Casey Piper <cpiper@codeaurora.org>