OvmfPkg: Switch timer in build time for OvmfPkg

OvmfPkg: Switch timer in build time for OvmfPkg

BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=3711

Discussion in https://bugzilla.tianocore.org/show_bug.cgi?id=1496 shows
that 8254TimerDxe was not written for OVMF. It was moved over from
PcAtChipsetPkg to OvmfPkg in 2019.  Probably because OVMF was the only
user left.

Most likely the reason OVMF used 8254TimerDxe initially was that it could
just use the existing driver in PcAtChipsetPkg.  And it simply hasn't
been changed ever.

CSM support was moved in 2019 too. (CSM support depends on 8254/8259
drivers). So 8254TimerDxe will be used when CSM_ENABLE=TRUE.

There are 4 .dsc which include the 8254Timer.
 - OvmfPkg/AmdSev/AmdSevX64.dsc
 - OvmfPkg/OvmfPkgIa32.dsc
 - OvmfPkg/OvmfPkgIa32X64.dsc
 - OvmfPkg/OvmfPkgX64.dsc

For the three OvmfPkg* configs using 8254TimerDxe with CSM_ENABLE=TRUE
and LapicTimerDxe otherwise.

For the AmdSev config it doesn't make sense to support a CSM. So use
the lapic timer unconditionally.

Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Brijesh Singh <brijesh.singh@amd.com>
Cc: Erdem Aktas <erdemaktas@google.com>
Cc: James Bottomley <jejb@linux.ibm.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Suggested-by: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>
Signed-off-by: Min Xu <min.m.xu@intel.com>