configs/infra: Add `ARM_TF_ARM_BL31_IN_DRAM` build control flag

BL31 can be loaded in DRAM or SRAM based on the value of
ARM_TF_ARM_BL31_IN_DRAM build option. Add ARM_TF_ARM_BL31_IN_DRAM in
the config files so that the build-option and corresponding value can
be passed as an argument while building trusted firmware.

Signed-off-by: Rohit Mathew <rohit.mathew@arm.com>
Change-Id: Ia7a6a253f82ca50a1200247fdd567d8105fe3c96