OvmfPkg: sync FADT with QEMU: PM Timer is 24-bit

Clear TMR_VAL_EXT.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>

git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@13616 6f19259b-4bc3-4df7-8a09-765794883524
MU SOURCE COMMIT:c2129fe67b89adbacb3e990c6baca974d7539495

Platforms/OvmfPkg/AcpiTables/Platform.h

@@ -58,7 +58,7 @@
 #define DAY_ALRM        0x00
 #define MON_ALRM        0x00
 #define CENTURY         0x00
-#define FLAG            EFI_ACPI_1_0_WBINVD | EFI_ACPI_1_0_PROC_C1 | EFI_ACPI_1_0_SLP_BUTTON | EFI_ACPI_1_0_RTC_S4 | EFI_ACPI_1_0_TMR_VAL_EXT
+#define FLAG            EFI_ACPI_1_0_WBINVD | EFI_ACPI_1_0_PROC_C1 | EFI_ACPI_1_0_SLP_BUTTON | EFI_ACPI_1_0_RTC_S4
 
 #endif