x86/sgx: Add SGX architectural data structures

Define the SGX architectural data structures used by various SGX
functions. This is not an exhaustive representation of all SGX data
structures but only those needed by the kernel.

The goal is to sequester hardware structures in "sgx/arch.h" and keep
them separate from kernel-internal or uapi structures.

The data structures are described in Intel SDM section 37.6.
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Acked-by: Jethro Beekman <jethro@fortanix.com>
Link: https://lkml.kernel.org/r/20201112220135.165028-2-jarkko@kernel.org