Claude review: PCI/P2PDMA: Add CONFIG_PCI_P2PDMA_CORE

[Claude Code Review Bot <claude-review@example.com>]

Patch Review

Splits `CONFIG_PCI_P2PDMA` into a core part (provider lookup) and the full feature (requires `ZONE_DEVICE`). This unblocks vfio-pci on 32-bit platforms.

**Observation**: The patch removes the `depends on 64BIT` from `PCI_P2PDMA` but the comment it also removes says:
```
-	#
-	# The need for the scatterlist DMA bus address flag means PCI P2PDMA
-	# requires 64bit
-	#
-	depends on 64BIT
```
This comment was explaining why `64BIT` was needed. Since `ZONE_DEVICE` itself already implies `64BIT` on most architectures (via `MEMORY_HOTPLUG`), the removal is likely fine in practice, but it would be worth confirming that `ZONE_DEVICE` is truly never selectable on 32-bit. If it were, the scatterlist DMA bus address flag concern would resurface.

**Minor**: `default n` is unnecessary for a bool Kconfig symbol -- `n` is already the default:
```
+config PCI_P2PDMA_CORE
+	default n
+	bool
```

The `#ifdef CONFIG_PCI_P2PDMA` guards in `p2pdma.c` look correct -- the file is now compiled under `_CORE`, and the full P2P functionality is conditionally compiled within it.

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