aarch32: optee: define the OP-TEE secure payload ...

AArch32 only platforms can boot the OP-TEE secure firmware as
a BL32 secure payload. Such configuration can be defined through
AARCH32_SP=optee.

The source files can rely on AARCH32_SP_OPTEE to condition
OP-TEE boot specific instruction sequences.

OP-TEE does not expect ARM Trusted Firmware formatted structure
as boot argument. Load sequence is expected to have already loaded
to OP-TEE boot arguments into the bl32 entrypoint info structure.

Last, AArch32 platform can only boot AArch32 OP-TEE images.

Change-Id: Ic28eec5004315fc9111051add6bb1a1d607fc815
Signed-off-by: Etienne Carriere <etienne.carriere@linaro.org>