CRIS: Allow arch dependent delay to override common version.

include/asm-cris/delay.h

···
 
 extern unsigned long loops_per_usec; /* arch/cris/mm/init.c */
 
+/* May be defined by arch/delay.h. */
+#ifndef udelay
 static inline void udelay(unsigned long usecs)
 {
 	__delay(usecs * loops_per_usec);
 }
+#endif
 
 #endif /* defined(_CRIS_DELAY_H) */