x86/amd_node: Remove dependency on AMD_NB

Cache the root devices locally so that there are no more dependencies on
AMD_NB.

Signed-off-by: Yazen Ghannam <yazen.ghannam@amd.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20241206161210.163701-13-yazen.ghannam@amd.com