dt-bindings: serial: samsung: add exynos7870-uart compatible

Document the compatible string for Exynos7870's UART driver. The
devicetree property samsung,uart-fifosize must be mandatory, as the
driver enquires about the FIFO sizes. This feature makes it compatible
with Exynos8895's UART.

Signed-off-by: Kaustabh Chakraborty <kauschluss@disroot.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20250219-exynos7870-uart-v2-1-c8c67f3a936c@disroot.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>