fix(plat/arm_fpga): enable AMU extension

As done recently for plat/tc0 in b5863cab

, enable AMU explicitly.
This is necessary as the recent changes that enable SVE for the secure
world disable AMU by default in the CPTR_EL3 reset value.

Change-Id: Ie3abf1dee8a4e1c8c39f934da8e32d67891f5f09
Signed-off-by: Tom Cosgrove <tom.cosgrove@arm.com>

plat/arm/board/arm_fpga/platform.mk

@@ -32,6 +32,8 @@ $(eval $(call add_define,FPGA_PRELOADED_DTB_BASE))
 FPGA_PRELOADED_CMD_LINE := 0x1000
 $(eval $(call add_define,FPGA_PRELOADED_CMD_LINE))
 
+ENABLE_AMU		:=	1
+
 # Treating this as a memory-constrained port for now
 USE_COHERENT_MEM	:=	0