drivers: serial: ns16550: use MMIO device depending on Kconfig option

Add a new selectable Kconfig option to decide wether the device driver
is a MMIO device or not. Previous to this patch, the decision was maded
based on the existence of a definition in <soc.h>. The design was
fragile, as code compiled anyway if the definition was not present.

All platforms/boards that had the definition in <soc.h> select the
Kconfig option in their respective defconfig files.

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