arm/smpboot: address violations of MISRA C:2012 Rule 8.2

Add missing parameter name, no functional change.

Signed-off-by: Federico Serafini <federico.serafini@bugseng.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>

xen/arch/arm/arm64/smpboot.c

@@ -10,7 +10,7 @@
 #include <asm/acpi.h>
 
 struct smp_enable_ops {
-        int             (*prepare_cpu)(int);
+        int             (*prepare_cpu)(int cpu);
 };
 
 static paddr_t cpu_release_addr[NR_CPUS];