misra: deviate violations of Rule 21.6

MISRA C Rule 21.6 states that "The Standard Library input/output
functions shall not be used".

Xen does not use the functions provided by the Standard Library,
uses -nostdlib to ensure this.
Xen implements a set of functions that share the same names as their
Standard Library equivalent. The implementation of these functions is
available in source form: if some undefined or unspecified behavior
does arise in the implementation, it falls under the jurisdiction of
other MISRA guidelines.

Update ECLAIR configuration to deviate violations of the rule,
tag it as clean and add it to the monitored set.

Updated docs/misra/deviations.rst accordingly.

Signed-off-by: Federico Serafini <federico.serafini@bugseng.com>
Signed-off-by: Dmytro Prokopchuk <dmytro_prokopchuk1@epam.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>

automation/eclair_analysis/ECLAIR/deviations.ecl

@@ -650,7 +650,8 @@ implements a set of functions that share the same names as their Standard Librar
 The implementation of these functions is available in source form, so the undefined, unspecified
 or implementation-defined behaviors contemplated by the C Standard do not apply.
 If some undefined or unspecified behavior does arise in the implementation, it
-falls under the jurisdiction of other MISRA rules."
+falls under the jurisdiction of other MISRA guidelines."
+-config=MC3A2.R21.6,reports+={deliberate, "any()"}
 -config=MC3A2.R21.9,reports+={deliberate, "any()"}
 -config=MC3A2.R21.10,reports+={deliberate, "any()"}
 -doc_end