i2c: exynos5: Mention Exynos850 and ExynosAutoV9 in Kconfig

I2C controller chosen by I2C_EXYNOS5 config option is also suitable for
Exynos850 and ExynosAutoV9 SoCs. State that specifically in I2C_EXYNOS5
symbol help section.

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Reviewed-by: Chanho Park <chanho61.park@samsung.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>