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>
diff --git a/board/intel/slimbootloader/MAINTAINERS b/board/intel/slimbootloader/MAINTAINERS
new file mode 100644
index 0000000..e693551
--- /dev/null
+++ b/board/intel/slimbootloader/MAINTAINERS
@@ -0,0 +1,6 @@
+Intel Slim Bootloader Payload
+M:	Aiden Park <aiden.park@intel.com>
+S:	Maintained
+F:	board/intel/slimbootloader
+F:	include/configs/slimbootloader.h
+F:	configs/slimbootloader_defconfig