(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 PCIe UART devices.
This fixes userspace console logging (without an UART device, there's no tty, only kernel prints work).