pch.rs: PchType enum spanning 12 PCH generations
from_generation(): maps IntelGeneration to PchType automatically
display_well_base(): per-PCH power well register base
has_separate_ddi_wells(): Gen9+ PCH feature flag
gmbus_base(): 0x5100 (pre-PCH) vs 0xC5100 (PCH-based)
ddi_port_count(): 2-6 ports per PCH generation
This gates correct DDI buffer, power well, and GMBUS configuration
per platform. Previously all platforms used Gen9 hardcoded defaults.
vasilito
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