Its API conforms to the docs. There are 16 IRQ channels available, which
will be used as assignable to the GPIO numbers. In most cases, the irq
channel is GPIO_no % 16.
Changes are:
- Have two separate tables for SAM21 and SAMD51.
- Use a short table for SAMD21.
- Add a comment to each line telling what it's for, making further use
easier.
- Add preliminary handlers/entries for PendSV, EIC and Sercom. These will
be replaced later when the respecitve modules are added.
