(continuing from https://reviews.freebsd.org/D19507)
`uart_bus_probe` matches discovered devices to system devices using resource addresses (`uart_cpu_eqres(&sc->sc_bas, &sysdev->bas)`).
The Amazon EC2 UART has different addresses in ACPI (`0x90A0000`) and PCI (`0x80118000`). Let's use the PCI address from the SPCR table to match PCI UART devices.
This fixes `/dev/console` (without an UART device, there's no tty, only kernel prints worked).