TXXXX: fix 'show hardware cpu' issue on arm64

Traceback (most recent call last):
  File "/usr/libexec/vyos/op_mode/cpu.py", line 76, in <module>
    res = vyos.opmode.run(sys.modules[__name__])
  File "/usr/lib/python3/dist-packages/vyos/opmode.py", line 200, in run
    res = func(**args)
  File "/usr/libexec/vyos/op_mode/cpu.py", line 58, in show
    cpu_data = _get_raw_data()
  File "/usr/libexec/vyos/op_mode/cpu.py", line 40, in _get_raw_data
    return vyos.cpu.get_cpus()
  File "/usr/lib/python3/dist-packages/vyos/cpu.py", line 83, in get_cpus
    cpus_dict = _find_physical_cpus()
  File "/usr/lib/python3/dist-packages/vyos/cpu.py", line 76, in _find_physical_cpus
    phys_cpus[num] = cpu[num]
NameError: name 'cpu' is not defined

python/vyos/cpu.py

@@ -73,7 +73,7 @@ def _find_physical_cpus():
             # On other architectures, e.g. on ARM, there's no such field.
             # We just assume they are different CPUs,
             # whether single core ones or cores of physical CPUs.
-            phys_cpus[num] = cpu[num]
+            phys_cpus[num] = cpus[num]
 
     return phys_cpus