Quickstart guide for SAM EIC driver

This is the quickstart guide for the SAM4L External Interrupt Controller (EIC) Driver, with step-by-step instructions on how to configure and use the driver in a selection of use cases.

The use cases contain several code fragments. The code fragments in the steps for setup can be copied into a custom initialization function, while the steps for usage can be copied into, e.g., the main application function.