Such constants are MCU specific so shouldn't be specified in the board
config file (else it leads to too much duplication of code).
This patch also adds I2C timing values for the F767/F769 for 100k, 400k
and 1MHz I2C bus frequencies.
There is a minor functional change with this patch, that the GPIO are now
configured in fast mode, whereas they were in high speed mode before. But
the SDIO should still work because SD CK frequency is at most 25MHz.
