doc: arch: Convert README.xtensa 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 2980b77..369d8ee 100644
--- a/doc/arch/index.rst
+++ b/doc/arch/index.rst
@@ -15,3 +15,4 @@
    sandbox
    sh
    x86
+   xtensa