Blackfin: document anomaly 05000234 workaround

Note the reason for using CHIPD over DSPID.
Signed-off-by: NRobin Getz <robin.getz@analog.com>
Signed-off-by: NMike Frysinger <vapier@gentoo.org>

arch/blackfin/include/asm/processor.h

@@ -131,8 +131,8 @@ unsigned long get_wchan(struct task_struct *p);
 /* Get the Silicon Revision of the chip */
 static inline uint32_t __pure bfin_revid(void)
 {
-	/* stored in the upper 4 bits */
-	uint32_t revid = bfin_read_CHIPID() >> 28;
+	/* Always use CHIPID, to work around ANOMALY_05000234 */
+	uint32_t revid = (bfin_read_CHIPID() & CHIPID_VERSION) >> 28;
 
 #ifdef CONFIG_BF52x
 	/* ANOMALY_05000357