dt-bindings: samsung,mipi-dsim: document exynos7870 DSIM compatible

Add compatible string for Exynos7870 DSIM bridge controller. The
device requires four clock sources, in schema they're named as "bus",
"pll", "byte", and "esc".

Suggested-by: Krzysztof Kozlowski <krzk@kernel.org>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Kaustabh Chakraborty <kauschluss@disroot.org>
Signed-off-by: Inki Dae <inki.dae@samsung.com>