Fix MPS availability check (#13947)

Co-authored-by: Rohit Gupta <[email protected]>

Author: akihironitta

src/pytorch_lightning/accelerators/mps.py

@@ -24,7 +24,7 @@
 
 # For using the `MPSAccelerator`, user's machine should have `torch>=1.12`, Metal programming framework and
 # the ARM-based Apple Silicon processors.
-_MPS_AVAILABLE = _TORCH_GREATER_EQUAL_1_12 and torch.backends.mps.is_available() and platform.platform() == "arm"
+_MPS_AVAILABLE = _TORCH_GREATER_EQUAL_1_12 and torch.backends.mps.is_available() and platform.processor() == "arm"
 
 
 class MPSAccelerator(Accelerator):