aemfvp-a: Update the value of secure memory

Update the fvp value of secure-memory to 1 in the FVP. This
enables TZC-400 security checking. Further information on
which can be found here:
https://developer.arm.com/documentation/ddi0504/c/introduction/about-the-tzc-400/tzc-400-overview



Signed-off-by: Ben Taylor <ben.taylor@linaro.org>
Change-Id: I52e45121c481ccd3706923b1db6d0f586a64d29d