x86, intel_txt: Factor out the code for S3 setup

S3 sleep requires special setup in tboot.  However, the data
structures needed to do such setup are only available if
CONFIG_ACPI_SLEEP is enabled.  Abstract them out as much as possible,
so we can have a single tboot_setup_sleep() which either is a proper
implementation or a stub which simply calls BUG().

Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Acked-by: Shane Wang <shane.wang@intel.com>
Cc: Joseph Cihula <joseph.cihula@intel.com>