feat(ast2700): add Aspeed AST2700 platform support

Aspeed AST2700 is a quad-core SoC with ARM Cortex-A35 integrated.
This patch adds the initial platform support for AST2700 and also
updates the documents.

Change-Id: I1796f7aae5ed2d1799e91fabb8949607959cd9b3
Signed-off-by: Chia-Wei Wang <chiawei_wang@aspeedtech.com>
diff --git a/docs/plat/index.rst b/docs/plat/index.rst
index 57c7303..188c986 100644
--- a/docs/plat/index.rst
+++ b/docs/plat/index.rst
@@ -8,6 +8,7 @@
 
    allwinner
    arm/index
+   ast2700
    meson-axg
    meson-gxbb
    meson-gxl