IntelFrameworkModulePkg: AcpiS3SaveDxe: call S3Ready() at End-of-Dxe

IntelFrameworkModulePkg: AcpiS3SaveDxe: call S3Ready() at End-of-Dxe

Call S3Ready() whenever the first of the following occurs:
- a driver signals End-of-Dxe,
- a driver calls EFI_ACPI_S3_SAVE_PROTOCOL.S3Save().

S3Ready() already contains a static, function scope "latch" that causes it
to exit early when called for the second time or later.

(At the moment, the only platform in the edk2 tree that includes this
driver is Vlv2TbltDevicePkg. That platform does not signal End-of-Dxe
(yet).)

http://thread.gmane.org/gmane.comp.bios.tianocore.devel/16088/focus=16146
Suggested-by: Yao Jiewen <jiewen.yao@intel.com>
Cc: Yao Jiewen <jiewen.yao@intel.com>
Cc: Jeff Fan <jeff.fan@intel.com>
Cc: David Wei <david.wei@intel.com>
Cc: Tim He <tim.he@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@17805 6f19259b-4bc3-4df7-8a09-765794883524