[amd64] Enable AMD_MEM_ENCRYPT

Hi,

It would be great to offer support for AMD Secure Memory Encryption (SME) in Bullseye. This provides the ability to mark individual pages of memory as encrypted using the standard x86 page tables. According to the documentation, the encryption of data is done with AES-128, and generated by an onboard NIST SP 800-90 compliant hardware random number generator. Also, the key is randomly generated on each system reset and is not visible to any software running on the CPU cores.