ARC: [SMP] ISS SMP extension bitrot

* Move extension specific code out of common SMP code
* Don't enable it by default for SMP

Signed-off-by: Vineet Gupta <vgupta@synopsys.com>

arch/arc/include/asm/irq.h

@@ -19,8 +19,6 @@
 #include <asm-generic/irq.h>
 
 extern void arc_init_IRQ(void);
-extern int get_hw_config_num_irq(void);
-
 void arc_local_timer_setup(unsigned int cpu);
 
 #endif