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.
This follows the pattern of other peripherals (I2C, SPI) to specify the
pins using pin objects instead of a pair of GPIO port and pin number. It
makes it easier to customise the UART pins for a particular board.
