i2c: qup: Add support for Fast Mode Plus

Previously the QUP driver limited operation mode to I2C Fast Mode. Add
Fast Mode Plus functionality by raising SCL limit from 400kHz to 1MHz.

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>