dt-bindings: display: Augment s6e63m0 bindings

This fixes the following problems with the s6e63m0 display
bindings:

- When used on the DSI bus, the panel is listed directly as
  a subnode on the DSI host so the "port" node is not
  compulsory. Remove "port" from required properties.

- The panel contains its own backlight control, so reference
  the backlight common properties and list default-brightness
  and max-brightness as supported but optional properties.

Cc: devicetree@vger.kernel.org
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210101114522.1981838-1-linus.walleij@linaro.org