i2c: designware: move i2c_dw_plat_prepare_clk to common

Move the i2c_dw_plat_prepare_clk funciton to common file in preparation
for its use also by the master driver.

Acked-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Phil Reid <preid@electromag.com.au>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>