soc/arm/nxp*: rearrange for standard use of extern "C"

Consistently place C++ use of extern "C" after all include directives,
within the negative branch of _ASMLANGUAGE if used.

Remove extern "C" support from files that don't declare objects or
functions.

Background from issue #17997:

Declarations that use C linkage should be placed within extern "C"
so the language linkage is correct when the header is included by
a C++ compiler.

Similarly #include directives should be outside the extern "C" to
ensure the language-specific default linkage is applied to any
declarations provided by the included header.

See: https://en.cppreference.com/w/cpp/language/language_linkage
Signed-off-by: Peter Bigot <peter.bigot@nordicsemi.no>

soc/arm/nxp_kinetis/k6x/soc.h

@@ -17,10 +17,6 @@
 
 #include <sys/util.h>
 
-#ifdef __cplusplus
-extern "C" {
-#endif
-
 /* default system clock */
 
 #define SYSCLK_DEFAULT_IOSC_HZ MHZ(120)
@@ -40,8 +36,4 @@ extern "C" {
 
 #endif /* !_ASMLANGUAGE */
 
-#ifdef __cplusplus
-}
-#endif
-
 #endif /* _SOC__H_ */