plat/arm/board: enable AMU for RD-N2

AMU counters are used for monitoring the CPU performance. RD-N2 platform
has architected AMU available for each core. Enable the use of AMU by
non-secure OS for supporting the use of counters for processor
performance control (ACPI CPPC).

Change-Id: I5cc749cf63c18fc5c7563dd754c2f42990a97e23
Signed-off-by: Pranav Madhu <pranav.madhu@arm.com>

plat/arm/board/rdn2/platform.mk

-# Copyright (c) 2020, ARM Limited and Contributors. All rights reserved.
+# Copyright (c) 2020-2021, ARM Limited and Contributors. All rights reserved.
 #
 # SPDX-License-Identifier: BSD-3-Clause
 #
@@ -57,3 +57,4 @@ NT_FW_CONFIG		:=	${BUILD_PLAT}/fdts/${PLAT}_nt_fw_config.dtb
 $(eval $(call TOOL_ADD_PAYLOAD,${NT_FW_CONFIG},--nt-fw-config))
 
 override CTX_INCLUDE_AARCH32_REGS	:= 0
+override ENABLE_AMU			:= 1