Documentation/x86: Document SGX kernel architecture

Documentation/x86: Document SGX kernel architecture

Document the Intel SGX kernel architecture. The fine-grained architecture
details can be looked up from Intel SDM Volume 3D.

Co-developed-by: Sean Christopherson <[email protected]>
Signed-off-by: Sean Christopherson <[email protected]>
Signed-off-by: Jarkko Sakkinen <[email protected]>
Signed-off-by: Borislav Petkov <[email protected]>
Acked-by: Jethro Beekman <[email protected]>
Cc: [email protected]
Link: https://lkml.kernel.org/r/[email protected]