x86: slimbootloader: Add a slimbootloader device tree

Add a new device tree which has very minimum nodes
- x86 reset
- x86 tsc_timer
- x86 pci
- Slim Bootloader serial

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>