Platform check incorrect on M1 pro device

[adamgayoso]

Excited to see this get in!

https://github.com/Lightning-AI/lightning/blob/0f6caffa577e2c8d9602dc8fba37c13e128b2120/src/pytorch_lightning/accelerators/mps.py#L27

On my device I see this:

In [5]: import platform
   ...: 

In [6]: platform.platform()
Out[6]: 'macOS-12.3.1-arm64-arm-64bit'

so _MPS_AVAILABLE is incorrectly false

cc @tchaton @rohitgr7 @akihironitta @justusschock