i2c: sprd: Make I2C driver can be built as a module

Now there is no need to keep our I2C driver to be initialized so early,
thus changing to module level and let it can be built as a module,
meanwhile adding some module information.

Signed-off-by: Baolin Wang <baolin.wang@linaro.org>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>