(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).