arch: x86: core: early_serial: obtain NS16550 uart base address from DT

The NS16550 UART base address was hardcoded in <soc.h> headers. This
bypasses the console choice defined in Devicetree. Hardcoded hardware
choices must be avoided now that DT is in place.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>