Add Port multiplexer data to SAM devices

[henrikssn]

Currently, a typical SAM gpio port looks like this:

<gpio port="A" pin="21">
  <signal driver="eic" name="extint5"/>
  <signal driver="gclk" name="io5"/>
  <signal driver="i2s" name="fs0"/>
  <signal driver="ptc" name="x9"/>
  <signal driver="sercom" instance="3" name="pad3"/>
  <signal driver="sercom" instance="5" name="pad3"/>
  <signal driver="tcc" instance="0" name="wo7"/>
</gpio>

We need to add the peripheral function of the port multiplexer (A-H) to this data, essentially encoding Table 7-1 (PORT Function Multiplexing) from the datasheet. See modm-io/modm#430 for an example where this would have been useful.

http://ww1.microchip.com/downloads/en/DeviceDoc/SAM_D21_DA1_Family_DataSheet_DS40001882F.pdf