Set DYNAMIC_WORKAROUND_CVE_2018_3639=1 on FVP by default

The upcoming patch that adds dynamic mitigation for Cortex-A76
requires that DYNAMIC_WORKAROUND_CVE_2018_3639=1.  On FVP, we pull in
all the CPU files into the build which means there will be a build
failure if DYNAMIC_WORKAROUND_CVE_2018_3639=0.

Change-Id: I2e781cbeafbf5d16eaabf76a1677e0c9f81269d2
Signed-off-by: Dimitris Papastamos <dimitris.papastamos@arm.com>

plat/arm/board/fvp/platform.mk

@@ -206,6 +206,9 @@ ENABLE_PLAT_COMPAT	:= 	0
 # Enable Activity Monitor Unit extensions by default
 ENABLE_AMU			:=	1
 
+# Enable dynamic mitigation support by default
+DYNAMIC_WORKAROUND_CVE_2018_3639	:=	1
+
 ifeq (${ENABLE_AMU},1)
 BL31_SOURCES		+=	lib/cpus/aarch64/cortex_a75_pubsub.c	\
 				lib/cpus/aarch64/cortex_ares_pubsub.c	\