OvmfPkg/OvmfXen: Set PcdFSBClock

OvmfPkg/OvmfXen: Set PcdFSBClock

Update gEfiMdePkgTokenSpaceGuid.PcdFSBClock so it can have the correct
value when SecPeiDxeTimerLibCpu start to use it for the APIC timer.

Currently, nothing appear to use the value in PcdFSBClock before
XenPlatformPei had a chance to set it even though TimerLib is included
in modules run before XenPlatformPei.

XenPlatformPei doesn't use any of the functions that would use that
value. No other modules in the PEI phase seems to use the TimerLib
before PcdFSBClock is set. There are currently two other modules in
the PEI phase that needs the TimerLib:
- S3Resume2Pei, but only because LocalApicLib needs it, but nothing is
  using the value from PcdFSBClock.
- CpuMpPei, but I believe it only runs after XenPlatformPei

Before the PEI phase, there's the SEC phase, and SecMain needs
TimerLib because of LocalApicLib. And it initialise the APIC timers
for the debug agent. But I don't think any of the DebugLib that
OvmfXen could use are actually using the *Delay functions in TimerLib,
and so would not use the value from PcdFSBClock which would be
uninitialised.

A simple runtime test showed that TimerLib doesn't use PcdFSBClock
value before it is set.

Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=2490
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Message-Id: <20210412133003.146438-8-anthony.perard@citrix.com>
[lersek@redhat.com: cast Freq to UINT32 for PcdSet32S(), not for ASSERT()]