x86: slimbootloader: Add serial driver

Slim Bootloader provides serial port info thru its HOB list pointer.
All these HOBs are eligible for Slim Bootloader based board only.
- Get serial port information from the serial port info HOB
- Leverage ns16550 driver with slimbootloader specific platform data

Signed-off-by: Aiden Park <aiden.park@intel.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>