doc: arch: Convert README.arm64 to reST

Convert plain text documentation to reStructuredText format and add
it to Sphinx TOC tree. No essential content change.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/doc/arch/index.rst b/doc/arch/index.rst
index f24b4a3..360b5d9 100644
--- a/doc/arch/index.rst
+++ b/doc/arch/index.rst
@@ -6,5 +6,6 @@
 .. toctree::
    :maxdepth: 2
 
+   arm64
    mips
    x86