[PATCH] docs/misra: add D4.12

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

Add D4.12 with the same explanation as the rules of the R21 series.
D4.12 refers to the standard library memory allocation functions and
similar third party libraries with memory allocation functions. It
doesn't refer to the in-tree implementation we have in Xen which is
subject to MISRA C rules and MISRA C scanning.

Signed-off-by: Stefano Stabellini <stefano.stabellini@xxxxxxx>

diff --git a/docs/misra/rules.rst b/docs/misra/rules.rst
index 80e5e972ad..bc8506add4 100644
--- a/docs/misra/rules.rst
+++ b/docs/misra/rules.rst
@@ -76,6 +76,11 @@ maintainers if you want to suggest a change.
        considered libraries from MISRA C point of view as they are
        imported in source form)
 
+   * - `Dir 4.12 
<https://gitlab.com/MISRA/MISRA-C/MISRA-C-2012/Example-Suite/-/blob/master/D_04_12.c>`_
+     - Required
+     - Dynamic memory allocation shall not be used
+     - Xen doesn't provide, use, or link against a Standard Library 
[#xen-stdlib]_
+
    * - `Dir 4.14 
<https://gitlab.com/MISRA/MISRA-C/MISRA-C-2012/Example-Suite/-/blob/master/D_04_14.c>`_
      - Required
      - The validity of values received from external sources shall be