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>
12 files changed