dt-bindings: i2c: exynos5: add exynos7870-hsi2c compatible

Exynos7870's HS-I2C controllers are entirely compatible with
samsung,exynos7-hsi2c. Document Exynos7870's HS-I2C compatible string
appropriately.

Signed-off-by: Kaustabh Chakraborty <[e-mail]>
Reviewed-by: Krzysztof Kozlowski <[e-mail]>
Signed-off-by: Andi Shyti <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

Documentation/devicetree/bindings/i2c/i2c-exynos5.yaml

···
       - items:
           - enum:
               - samsung,exynos5433-hsi2c
+              - samsung,exynos7870-hsi2c
               - tesla,fsd-hsi2c
           - const: samsung,exynos7-hsi2c
       - items: