[XBOX][FREELDR] Initialize PCI-AGP bridge BAR values

This fixes problem with Xbox GPU not being detected as a children device.

CORE-17600 CORE-16216

[FREELDR][HALXBOX] Initialize PCI bridge secondary & subordinate bus values

This fixes the recursion when pci.sys attempts to get PCI bridge children
devices, which in fact are non-existing.

Note t

[FREELDR][HALXBOX] Initialize PCI bridge secondary & subordinate bus values

This fixes the recursion when pci.sys attempts to get PCI bridge children
devices, which in fact are non-existing.

Note that the problem in pci.sys itself also deserves a separate fix.

CORE-16319 CORE-16216

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[FREELDR] Replace CONFIG_CMD macro with a straightforward bitfield struct

Also fix magic values related to PCI registers.

[XBOX] Move NV2A XGPU definitions into header file

TODO: helper functions should be also moved. CORE-16216

[FREELDR] Fix register offset

Addendum to 10e52f6.

[FREELDR] Fix some more register accesses and magic values

Addendum to 764e884.

[FREELDR] Restructure platform-specific code. No code changes.