Blackfin: allow NMI watchdog to be used w/RETN as a scratch reg

NMIs are not safe to return from because many anomaly workarounds are
implemented by disabling interrupts.  The NMI obviously violates this
assumption.  Since the NMI watchdog never returns, we don't have to
worry about it clobbering RETN when it is being used as a scratch register
with the exception stack.
Signed-off-by: NGraf Yang <graf.yang@analog.com>
Signed-off-by: NMike Frysinger <vapier@gentoo.org>

arch/blackfin/Kconfig.debug

@@ -238,7 +238,7 @@ config EARLY_PRINTK
 config NMI_WATCHDOG
 	bool "Enable NMI watchdog to help debugging lockup on SMP"
 	default n
-	depends on (SMP && !BFIN_SCRATCH_REG_RETN)
+	depends on SMP
 	help
 	  If any CPU in the system does not execute the period local timer
 	  interrupt for more than 5 seconds, then the NMI handler dumps debug