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_imx/mcimx6x_m4/soc.h

@@ -7,10 +7,6 @@
 #ifndef _SOC__H_
 #define _SOC__H_
 
-#ifdef __cplusplus
-extern "C" {
-#endif
-
 #ifndef _ASMLANGUAGE
 
 #include "rdc.h"
@@ -20,8 +16,4 @@ extern "C" {
 
 #endif /* !_ASMLANGUAGE */
 
-#ifdef __cplusplus
-}
-#endif
-
 #endif /* _SOC__H_ */

soc/arm/nxp_imx/mcimx7_m4/soc.h

@@ -7,10 +7,6 @@
 #ifndef _SOC__H_
 #define _SOC__H_
 
-#ifdef __cplusplus
-extern "C" {
-#endif
-
 #ifndef _ASMLANGUAGE
 
 #include "rdc.h"
@@ -21,8 +17,4 @@ extern "C" {
 
 #endif /* !_ASMLANGUAGE */
 
-#ifdef __cplusplus
-}
-#endif
-
 #endif /* _SOC__H_ */

soc/arm/nxp_imx/rt/soc.h

@@ -9,10 +9,6 @@
 
 #include <sys/util.h>
 
-#ifdef __cplusplus
-extern "C" {
-#endif
-
 #ifndef _ASMLANGUAGE
 
 #include <fsl_common.h>
@@ -23,6 +19,10 @@ extern "C" {
  */
 #include <kernel_includes.h>
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 #if defined(CONFIG_DISK_ACCESS_USDHC1) ||	\
 	defined(CONFIG_DISK_ACCESS_USDHC2)
 
@@ -36,10 +36,10 @@ void imxrt_usdhc_pinmux_cb_register(usdhc_pin_cfg_cb cb);
 
 #endif
 
-#endif /* !_ASMLANGUAGE */
-
 #ifdef __cplusplus
 }
 #endif
 
+#endif /* !_ASMLANGUAGE */
+
 #endif /* _SOC__H_ */