i2c: exynos: Update Kconfig documentation

Update the help text to reflect current support devices:
1. The Exynos high speed I2C driver supports Exynos5 (ARMv7), Exynos5433
   and Exynos7 (both ARMv8) SoCs,
2. The S3C I2C driver supports S3C, S5Pv210 and Exynos{3,4,5} SoCs.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>