fvp: Disable SYSTEM_SUSPEND when ARM_BL31_IN_DRAM

fvp: Disable SYSTEM_SUSPEND when ARM_BL31_IN_DRAM

After returning from SYSTEM_SUSPEND state, BL31 reconfigures the
TrustZone Controller during the boot sequence. If BL31 is placed in
TZC-secured DRAM, it will try to change the permissions of the memory it
is being executed from, causing an exception.

The solution is to disable SYSTEM_SUSPEND when the Trusted Firmware has
been compiled with ``ARM_BL31_IN_DRAM=1``.

Change-Id: I96dc50decaacd469327c6b591d07964726e58db4
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>