x86, acpi, tboot: Have a ACPI sleep override instead of calling tboot_sleep.

x86, acpi, tboot: Have a ACPI sleep override instead of calling tboot_sleep.

The ACPI suspend path makes a call to tboot_sleep right before
it writes the PM1A, PM1B values. We replace the direct call to
tboot via an registration callback similar to __acpi_register_gsi.

CC: Thomas Gleixner <tglx@linutronix.de>
CC: "H. Peter Anvin" <hpa@zytor.com>
CC: x86@kernel.org
CC: Len Brown <len.brown@intel.com>
CC: Joseph Cihula <joseph.cihula@intel.com>
CC: Shane Wang <shane.wang@intel.com>
CC: xen-devel@lists.xensource.com
CC: linux-pm@lists.linux-foundation.org
CC: tboot-devel@lists.sourceforge.net
CC: linux-acpi@vger.kernel.org
[v1: Added __attribute__ ((unused))]
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>