I've just been scoping the I2C clock line on my board and have noticed the following.
If HW_I2C_TIMING0 and HW_I2C_TIMING1 are set to 0x000f0007 and 0x001f000f for 400kHz operation as per page 1756 of the reference manual then the clock rate is 436kHz.
After reading the description I tried 0x0018000c in HW_I2C_TIMING0 to get a 1us clock high time, i.e. 24 clocks of 24MHz. I put 0x00240010 in HW_I2C_TIMING1 to get 1.5us clock low. 2.5us -> 400kHz.
Unfortunately this gave me 348kHz.
Trial and error gave me 400kHz by putting the following values in the registers:
0x0010000c -> HW_I2C_TIMING0 which scopes out as 1us clock high.
0x00230010 -> HW_I2C_TIMING1 which scopes out as 1.5us clock low.
Hope this is helpful.