x86: Allow locating the UART from ACPI tables

When coreboot does not pass a UART in its sysinfo struct, there is no
easy way to find it out.

Since coreboot does not actually init the serial device when serial is
disabled, it is not possible to make it add this information to the
sysinfo table.

Add a way to obtain this information from the DBG2 ACPI table, which is
normally set up by coreboot.

For now this only supports a memory-mapped 16550-style UART.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>