riscv: Add AST2700 SoC initial platform support

AST2700 SoCs integrates a Ibex 32-bits RISC-V core as the boot MCU
for the first stage bootloader execution, namely SPL.

This patch implements the preliminary base to successfully run SPL
on this RV32-based MCU to the console banner message.

Signed-off-by: Chia-Wei Wang <chiawei_wang@aspeedtech.com>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>
diff --git a/doc/board/aspeed/index.rst b/doc/board/aspeed/index.rst
new file mode 100644
index 0000000..d784c88
--- /dev/null
+++ b/doc/board/aspeed/index.rst
@@ -0,0 +1,9 @@
+.. SPDX-License-Identifier: GPL-2.0+
+
+Aspeed
+======
+
+.. toctree::
+   :maxdepth: 2
+
+   ibex-ast2700