samd/pin_af: Add the pin af table and its helper functions.

The pin af table is a representation of the MUX table from the data sheet.
It provides information for each pin about the supported device functions.

That information is needed by pin.irq, machine.ADC, machine.PWM,
machine.UART, machine.SPI and machine.I2C.  For each of these, the table
tells for each pin, which device number, af number and pad number is
assigned.  Using the table gives a straight, uniform access to the
information, where the benefit outweights the size of the table, which is
not that large.

The tables are MCU-specific.  It is not required to tell for each board,
which and where each of the above devices is available.  That makes addding
boards easy.

Note: The information for DAC and I2S was not included, since it affects
only a few pins.

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