dt-bindings: exynos_dsim: update of graph bindings

Of-graph bindings should describe ports present in the device, not the
devices it can be connected to. The patch replaces verbose description
with shorter but more precise one. While at it clock related properties
are moved to the main node as it is their actual location.

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>