docs/specs: initial spec summary for Ultravisor-related hcalls

docs/specs: initial spec summary for Ultravisor-related hcalls

For now this only covers hcalls relating to TPM communication since
it's the only one particularly important from a QEMU perspective atm,
but others can be added here where it makes sense.

The full specification for all hcalls/ucalls will eventually be made
available in the public/OpenPower version of the PAPR specification.

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Message-Id: <20190717205842.17827-2-mdroth@linux.vnet.ibm.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>