dt-bindings: display: bridge: tc358867: Document default DP preemphasis

Document default DP port preemphasis configurable via new DT property
"toshiba,pre-emphasis". This is useful in case the DP link properties
are known and starting link training from preemphasis setting of 0 dB
is not useful. The preemphasis can be set separately for both DP lanes
in range 0=0dB, 1=3.5dB, 2=6dB .

This is an endpoint property, not a port property, because the TC9595
datasheet does mention that the DP might operate in some sort of split
mode, where each DP lane is used to feed one display, so in that case
there might be two endpoints.

Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20240708150130.54484-1-marex@denx.de