SecurityPkg: Add support for RngDxe on AARCH64

AARCH64 support has been added to BaseRngLib via the optional
ARMv8.5 FEAT_RNG.

Refactor RngDxe to support AARCH64, note support for it in the
VALID_ARCHITECTURES line of RngDxe.inf and enable it in SecurityPkg.dsc.

Signed-off-by: Rebecca Cran <rebecca@nuviainc.com>
Reviewed-by: Liming Gao <gaoliming@byosoft.com.cn>
Acked-by: Jiewen Yao <Jiewen.yao@intel.com>
Reviewed-by: Sami Mujawar <sami.mujawar@arm.com>

SecurityPkg/RandomNumberGenerator/RngDxe/Rand/RdRand.h

@@ -0,0 +1,43 @@
+/** @file
+  Header for the RDRAND APIs used by RNG DXE driver.
+
+  Support API definitions for RDRAND instruction access, which will leverage
+  Intel Secure Key technology to provide high-quality random numbers for use
+  in applications, or entropy for seeding other random number generators.
+  Refer to http://software.intel.com/en-us/articles/intel-digital-random-number
+  -generator-drng-software-implementation-guide/ for more information about Intel
+  Secure Key technology.
+
+Copyright (c) 2013, Intel Corporation. All rights reserved.<BR>
+(C) Copyright 2015 Hewlett Packard Enterprise Development LP<BR>
+SPDX-License-Identifier: BSD-2-Clause-Patent
+
+**/
+
+#ifndef __RD_RAND_H__
+#define __RD_RAND_H__
+
+#include <Library/BaseLib.h>
+#include <Library/BaseMemoryLib.h>
+#include <Library/UefiBootServicesTableLib.h>
+#include <Library/TimerLib.h>
+#include <Protocol/Rng.h>
+
+/**
+  Generate high-quality entropy source through RDRAND.
+
+  @param[in]   Length        Size of the buffer, in bytes, to fill with.
+  @param[out]  Entropy       Pointer to the buffer to store the entropy data.
+
+  @retval EFI_SUCCESS        Entropy generation succeeded.
+  @retval EFI_NOT_READY      Failed to request random data.
+
+**/
+EFI_STATUS
+EFIAPI
+RdRandGenerateEntropy (
+  IN UINTN         Length,
+  OUT UINT8        *Entropy
+  );
+
+#endif  // __RD_RAND_H__