I2C: Verify at compile-time that pins have been set to the right mode

[hannobraun]

PIO0_10/PIO0_11 need to be set to standard/fast mode in their respective IOCON registers for them to be used with the I2C API. This is the default setting, and currently the I2C API doesn't verify that this hasn't been changed.

I'm not at all sure what this should look like in practice, but the various IOCON settings of each pin should probably be tracked at compile-time, and the I2C API should require the setting to be set correctly.