Recognize the Amazon PCI serial device found in a1.* EC2 instances
as an NS8250 UART.

This is the same as the UART found in EC2 "bare metal" instances,
except that the card vendor shows up as 0x0000 rather than 0x1d0f.
This seems like a bug in the EC2 firmware; but we might as well support
it anyway.

Reported by: Greg V