ARM: bL_switcher: do not hardcode GIC IDs in the code

Currently, GIC IDs are hardcoded making the code dependent on the 4+4 b.L
configuration.  Let's allow for GIC IDs to be discovered upon switcher
initialization to support other b.L configurations such as the 1+1 one,
or 2+3 as on the VExpress TC2.

Signed-off-by: Nicolas Pitre <nico@linaro.org>

include/linux/irqchip/arm-gic.h

@@ -75,6 +75,7 @@ static inline void gic_init(unsigned int nr, int start,
 	gic_init_bases(nr, start, dist, cpu, 0, NULL);
 }
 
+int gic_get_cpu_id(unsigned int cpu);
 void gic_migrate_target(unsigned int new_cpu_id);
 
 #endif /* __ASSEMBLY */