misc: Add Nitro Secure Module driver

When running Linux inside a Nitro Enclave, the hypervisor provides a
special virtio device called "Nitro Security Module" (NSM). This device
has 3 main functions:

1) Provide attestation reports
2) Modify PCR state
3) Provide entropy

This patch adds a driver for NSM that exposes a /dev/nsm device node which
user space can issue an ioctl on this device with raw NSM CBOR formatted
commands to request attestation documents, influence PCR states, read
entropy and enumerate status of the device. In addition, the driver
implements a hwrng backend.

Originally-by: Petre Eftime <petre.eftime@gmail.com>
Signed-off-by: Alexander Graf <graf@amazon.com>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20231011213522.51781-1-graf@amazon.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>