UefiPayloadPkg: Update ReadUnaligned64 in ACPI parsing

According to ACPI Specification, 64 bit physical address of the XSDT
provides indentical functionality to the RSDT but accommodates physical
address of description headers that are larger than 32 bits.

In this case physical address of XSDT table is 64 bit aligned, however
size of ACPI description tabled header is not 64 bit aligned. It leads
to the entry of other description headers are not 64 bit aligned. In
AARCH64 architecture, deference non-aligned 64 bit address to fetch
64-bit data will trigger Alignment fault. Use ReadUnaligned64 method
to fix this unaligned data access issue.

Signed-off-by: Ajan Zhong <ajan.zhong@newfw.com>

UefiPayloadPkg/UefiPayloadEntry/AcpiTable.c

@@ -77,7 +77,7 @@ ParseAcpiInfo (
     Entry64    = (UINT64 *)(Xsdt + 1);
     Entry64Num = (Xsdt->Length - sizeof (EFI_ACPI_DESCRIPTION_HEADER)) >> 3;
     for (Idx = 0; Idx < Entry64Num; Idx++) {
-      Signature = (UINT32 *)(UINTN)Entry64[Idx];
+      Signature = (UINT32 *)(UINTN)ReadUnaligned64 (&Entry64[Idx]);
       if (*Signature == EFI_ACPI_3_0_FIXED_ACPI_DESCRIPTION_TABLE_SIGNATURE) {
         Fadt = (EFI_ACPI_3_0_FIXED_ACPI_DESCRIPTION_TABLE *)Signature;
         DEBUG ((DEBUG_INFO, "Found Fadt in Xsdt\n"));