[arch][[arm64] add a function to translate cpu number to the cpu's ID

Will be used in GICv3 to send IPIs.

arch/arm64/include/arch/arm64/mp.h

@@ -42,4 +42,7 @@ static inline uint arch_curr_cpu_num(void) {
     return pc->cpu_num;
 }
 
+// Translate a CPU number back to the MPIDR of the CPU.
+uint64_t arm64_cpu_num_to_mpidr(uint cpu_num);
+
 __END_CDECLS

arch/arm64/mp.c

@@ -11,12 +11,12 @@
 #include <arch/mp.h>
 #include <arch/ops.h>
 #include <assert.h>
+#include <inttypes.h>
 #include <lk/err.h>
 #include <lk/init.h>
 #include <lk/main.h>
 #include <lk/trace.h>
 #include <platform/interrupts.h>
-#include <inttypes.h>
 #include <stdlib.h>
 #include <string.h>
 
@@ -143,6 +143,20 @@ void arm64_secondary_entry(ulong asm_cpu_num) {
 }
 #endif // WITH_SMP
 
+uint64_t arm64_cpu_num_to_mpidr(uint cpu_num) {
+#if WITH_SMP
+    if (cpu_num == 0) {
+        return boot_percpu.mpidr;
+    } else if (unlikely(cpu_num > secondaries_to_init)) {
+        return UINT64_MAX;
+    } else {
+        return secondary_percpu[cpu_num - 1].mpidr;
+    }
+#else
+    return boot_percpu.mpidr;
+#endif
+}
+
 void arm64_mp_init(void) {
 #if WITH_SMP
     arm64_mp_init_percpu();