i2c: designware: add support of platform data to set I2C mode

Use the platform data to set the clk_freq when there is no DT configuration
available. The clk_freq in turn will determine the I2C speed mode.

In Quark, there is currently no other configuration mechanism other than
board files.

Signed-off-by: Raymond Tan <raymond.tan@intel.com>
Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Reviewed-by: Hock Leong Kweh <hock.leong.kweh@intel.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>