x86/cc: Move arch/x86/{kernel/cc_platform.c => coco/core.c}

Move cc_platform.c to arch/x86/coco/. The directory is going to be the
home space for code related to confidential computing.

Intel TDX code will land here. AMD SEV code will also eventually be
moved there.

No functional changes.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
Link: https://lore.kernel.org/r/20220222185740.26228-3-kirill.shutemov@linux.intel.com