x86/sev: Rename mem_encrypt.c to mem_encrypt_amd.c

Both Intel TDX and AMD SEV implement memory encryption features. But the
bulk of the code in mem_encrypt.c is AMD-specific. Rename the file to
mem_encrypt_amd.c. A subsequent patch will extract the parts that can be
shared by both TDX and AMD SEV/SME into a generic file.

No functional changes.

Signed-off-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Tony Luck <tony.luck@intel.com>
Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
Tested-by: Tom Lendacky <thomas.lendacky@amd.com>
Link: https://lore.kernel.org/r/20211206135505.75045-3-kirill.shutemov@linux.intel.com