x86: serial: Use NS16550_DYNAMIC in Slim Bootloader

Slim Bootloader provides serial port info in its HOB to support
both IO or MMIO serial ports, but it's controlled by SYS_NS16550_MEM32
or SYS_NS16550_PORT_MAPPED in U-Boot.
To support both serial port configurations dynamically at runtime,
Slim Bootloader serial driver leverages NS16550_DYNAMIC.

Signed-off-by: Aiden Park <aiden.park@intel.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
[bmeng: remove the obsolete comments for data->type]
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>