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>