spdm: Allow control of next requester nonce through sysfs

Remote attestation services may mistrust the kernel to always use a
fresh nonce for SPDM authentication.

So allow user space to set the next requester nonce by writing to a
sysfs attribute.

Signed-off-by: Lukas Wunner <lukas@wunner.de>
Cc: James Bottomley <James.Bottomley@HansenPartnership.com>
Cc: Jérôme Glisse <jglisse@google.com>
Cc: Jason Gunthorpe <jgg@nvidia.com>