All board pins that have UART's assigned can be used. Baud rate range is
75 Baud to ~2 MBaud.
No flow control yet, and only RX is buffered. TX buffer and flow control
may be added later for SAMD51 with its larger RAM and Flash.
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.
