doc: Recommend using C rather than assembly language

Add a section for that in the coding guidelines.

Change-Id: Ie6819c4df5889a861460eb96acf2bc9c0cfb494e
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>

docs/process/coding-guidelines.rst

@@ -441,6 +441,25 @@ unsigned integer on all systems, cast it to ``unsigned int``.
 
 These guidelines should be updated if additional types are needed.
 
+Favor C language over assembly language
+---------------------------------------
+
+Generally, prefer code written in C over assembly. Assembly code is less
+portable, harder to understand, maintain and audit security wise. Also, static
+analysis tools generally don't analyze assembly code.
+
+There are, however, legitimate uses of assembly language. These include:
+
+  - Early boot code executed before the C runtime environment is setup.
+
+  - Exception handling code.
+
+  - Low-level code where the exact sequence of instructions executed on the CPU
+    matters, such as CPU reset sequences.
+
+  - Low-level code where specific system-level instructions must be used, such
+    as cache maintenance operations.
+
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