board: intel: Add new slimbootloader board

Add slimbootloader board to run U-boot as a Slim Bootloader payload
- Add new board/intel/slimbootloader directory with minimum codes
- Add slimbootloader configuration files
- Add doc/board/intel/slimbootloader.rst

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>
[bmeng: add slimbootloader board MAINTAINERS file]
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>