drivers/s390/char: Add Ultravisor io device

This patch adds a new miscdevice to expose some Ultravisor functions
to userspace. Userspace can send IOCTLs to the uvdevice that will then
emit a corresponding Ultravisor Call and hands the result over to
userspace. The uvdevice is available if the Ultravisor Call facility is
present.
Userspace can call the Retrieve Attestation Measurement
Ultravisor Call using IOCTLs on the uvdevice.

The uvdevice will do some sanity checks first.
Then, copy the request data to kernel space, build the UVCB,
perform the UV call, and copy the result back to userspace.

Signed-off-by: Steffen Eiden <[email protected]>
Reviewed-by: Janosch Frank <[email protected]>
Reviewed-by: Claudio Imbrenda <[email protected]>
Link: https://lore.kernel.org/kvm/[email protected]/
Message-Id: <[email protected]>
Signed-off-by: Janosch Frank <[email protected]> (whitespace and  tristate fixes, pick)

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