cdx: Split mcdi.h and reorganize headers

cdx: Split mcdi.h and reorganize headers

Move bitfield.h from the CDX controller directory to include/linux/cdx to make
them accessible to other drivers.

As part of this refactoring, split mcdi.h into two headers:

- mcdi.h: retains interface-level declarations
- mcdid.h: contains internal definitions and macros

This is in preparation for VersalNET EDAC driver that relies on it.

Signed-off-by: Shubhrajyoti Datta <shubhrajyoti.datta@amd.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Acked-by: Nikhil Agarwal <nikhil.agarwal@amd.com>
Link: https://lore.kernel.org/20250908115649.22903-1-shubhrajyoti.datta@amd.com