cellular: AT: fix build on ARMCC5

ARMC5 failed to compile the code with debug-profile (!!) as va_list
is getting into std:: namespace when one includes <cstdarg>. Other
compilers seem to be more relaxed, and so is ARMC5 if compiled
with other profiles.

Add the explicit std:: to references of va_list.

While here, remove one extra copy of "#include "PlatformMutex.h""
and a "#include <stdarg.h>" which is kind of duplicate of
"#include <cstdarg>".

Error being fixed:
--8<--8<--8<--
Compile [ 81.8%]: ATHandler.cpp
[Error] ATHandler.h@552,0:  #20: identifier "va_list" is undefined
[Error] ATHandler.cpp@1226,0:  #147: declaration is incompatible with "void mbed::ATHandler::handle_args(const char *, <error-type>)"  (declared at line 552 of "./mbed-os/features/cellular/framework/AT/ATHandler.h")
[ERROR] "./mbed-os/features/cellular/framework/AT/ATHandler.h", line 552: Error:  #20: identifier "va_list" is undefined
"./mbed-os/features/cellular/framework/AT/ATHandler.cpp", line 1226: Error:  #147: declaration is incompatible with "void mbed::ATHandler::handle_args(const char *, <error-type>)"  (declared at line 552 of "./mbed-os/features/cellular/framework/AT/ATHandler.h")
./mbed-os/features/cellular/framework/AT/ATHandler.cpp: 0 warnings, 2 errors

features/cellular/framework/AT/ATHandler.cpp

@@ -26,7 +26,6 @@
 #include "rtos/ThisThread.h"
 #include "Kernel.h"
 #include "CellularUtil.h"
-#include <stdarg.h>
 #include "SingletonPtr.h"
 
 using namespace mbed;
@@ -1223,7 +1222,7 @@ void ATHandler::cmd_start(const char *cmd)
     _cmd_start = true;
 }
 
-void ATHandler::handle_args(const char *format, va_list list)
+void ATHandler::handle_args(const char *format, std::va_list list)
 {
     while (*format != '\0') {
         if (*format == 'd') {

features/cellular/framework/AT/ATHandler.h

@@ -24,8 +24,8 @@
 #include "PlatformMutex.h"
 #include "nsapi_types.h"
 
-#include "PlatformMutex.h"
 #include "Callback.h"
+
 #include <cstdarg>
 
 namespace mbed {
@@ -549,7 +549,7 @@ class ATHandler {
 
 private:
     //Handles the arguments from given variadic list
-    void handle_args(const char *format, va_list list);
+    void handle_args(const char *format, std::va_list list);
 
     //Starts an AT command based on given parameters
     void handle_start(const char *cmd, const char *cmd_chr);