dt-bindings: display: sunxi: Improve endpoint ID scheme readability

The explanation for the endpoint ID numbering scheme is convoluted
and hard to understand.

This patch aims to improve the readability of it by combining the
existing two paragraphs, while also providing a diagram example,
and how endpoints should be numbered based on that example.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>