arch: c6x: platforms: include "asm/special_insns.h" to pass compiling

Include "asm/special_insns.h" to pass compiling.

The related error (with allmodconfig):
arch/c6x/platforms/plldata.c: In function ‘c6472_setup_clocks’:
arch/c6x/platforms/plldata.c:279:2: error: implicit declaration of function ‘get_coreid’ [-Werror=implicit-function-declaration]

---
 arch/c6x/platforms/plldata.c |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)

index 755359e..ddd1441 100644
@@ -20,6 +20,7 @@
 #include <asm/clock.h>
 #include <asm/setup.h>
 #include <asm/irq.h>
+#include <asm/special_insns.h>
 
 /*
  * Common SoC clock support.

1.7.7.6