x86/acpi/sleep: Provide registration for acpi_suspend_lowlevel.

Which by default will be x86_acpi_suspend_lowlevel.
This registration allows us to register another callback
if there is a need to use another platform specific callback.

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: linux-pm@lists.linux-foundation.org
CC: linux-acpi@vger.kernel.org
CC: Len Brown <len.brown@intel.com>
Signed-off-by: Liang Tang <liang.tang@oracle.com>
[v1: Fix when CONFIG_ACPI_SLEEP is not set]
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>