fsl_i2c: Add CONFIG_FSL_I2C_CUSTOM_{DFSR/FDR}

Some boards need a higher DFSR value than the spec currently
recommends so give these boards the means to define there own.

For completeness, add CONFIG_FSL_I2C_CUSTOM_FDR too.

Signed-off-by: Joakim Tjernlund <Joakim.Tjernlund@transmode.se>