doc: arch: Convert README.m68k 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 93fbb7e..ee90ec1 100644
--- a/doc/arch/index.rst
+++ b/doc/arch/index.rst
@@ -8,6 +8,7 @@
 
    arc
    arm64
+   m68k
    mips
    nds32
    nios2