Blackfin arch: Add ANOMALY_05000368 workaround

Possible RETS Register Corruption when Subroutine Is under 5 Cycles in Duration
Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Bryan Wu <cooloney@kernel.org>

arch/blackfin/kernel/entry.S

@@ -64,6 +64,11 @@ ENDPROC(_ret_from_fork)
 
 ENTRY(_sys_fork)
 	r0 = -EINVAL;
+#if (ANOMALY_05000371)
+	nop;
+	nop;
+	nop;
+#endif
 	rts;
 ENDPROC(_sys_fork)