i2c: exynos5: add support for HSI2C on Exynos5260 SoC

HSI2C module on Exynos5260 differs from current modules in
following ways:
1.  HSI2C on Exynos5260 has fifo_depth of 16bytes
2.  Module needs to be reset as a part of init sequence.

Hence, Following changes are involved.
1. Add a new compatible string and Updates the Documentation dt bindings.
2. Introduce a variant struct to support the changes in H/W
3. Reset the module during init. Thus, bringing the module back
to default state irrespective of what firmware did with it.

Signed-off-by: Naveen Krishna Chatradhi <ch.naveen@samsung.com>
Signed-off-by: Pankaj Dubey <pankaj.dubey@samsung.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>