Add support for EC2 i3.metal serial port

[cperciva]

Summary

Recognize the Amazon PCI serial device found in i3.metal EC2 instances
as an NS8250 UART.

Test Plan

Booting an EC2 i3.metal instance with this patch, uart2 shows up and if I write to it (e.g., cat /COPYRIGHT > /dev/ttyu2) that shows up in the EC2 Console Logs.

Unfortunately I haven't been able to figure out how to get the kernel to use this for its console output, which is kind of the point... any help will be much appreciated.

[cperciva]

Adding some reviewers who have touched this recently and may be able to tell me if I'm doing this right and/or what I'm missing.

[sbruno]

[imp]

sys/dev/uart/uart_bus_pci.c

128 ↗

(On Diff #48241)

It's traditional to also add this to puc, iirc.