i2c: qup: Add command-line parameter to override SCL frequency

Add a module parameter to override SCL frequency provided by firmware.
This can be useful when testing spec compliance for I2C modes or when
debugging issues across multiple operating frequencies.

Signed-off-by: Austin Christ <austinwc@codeaurora.org>
Reviewed-by: Sricharan R <sricharan@codeaurora.org>
Reviewed-by: Andy Gross <andy.gross@linaro.org>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>