doc: switch from CPP to C domain for .c files

Older Breathe versions didn't work correctly with the C domain, so there
was a mismatch when refering to functions and #defines, using :cpp: or
:c: depending on the origin of the refered symbol. With Breathe >=4.19
the C domain works correctly so all symbols can be refered to with the
:c: role.

Signed-off-by: Fabio Utzig <fabio.utzig@nordicsemi.no>

doc/conf.py

@@ -516,12 +516,10 @@ breathe_projects = {
 }
 breathe_default_project = "Zephyr"
 
-# Changing breathe configuration to force "c" domain doesn't work
-# see https://github.com/michaeljones/breathe/issues/282
-#breathe_domain_by_extension = {
-#    "h" : "c",
-#    "c" : "c",
-#    }
+breathe_domain_by_extension = {
+    "h": "c",
+    "c": "c",
+}
 
 # Qualifiers to a function are causing Sphihx/Breathe to warn about
 # Error when parsing function declaration and more.  This is a list