qemu/optee: load OP-TEE pageable part 2MB above OP-TEE image

OP-TEE dedicates the end of the Qemu secure DRAM as specific out-of-TEE
secure RAM. To support this configuration the trusted firmware should
not load OP-TEE resources in this area.

To overcome the issue, OP-TEE pageable image is now loaded 2MByte above
the secure RAM base address.

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