platform/x86: amd-pmc: put pmc suspend to delay workqueue

In the s2idle the PMC suspend invoked before some device noirq suspend.
In this case the XHCI noirq suspend was behind the PMC suspend so the
OS_HINT issued before some peripheral device reach its min sleep level.
Therefore put the PMC suspend to a delay work will can make sure OS_HINT issued
at right moment.

Signed-off-by: Prike Liang <Prike.Liang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>