Add support for booting qemu with TFA and optee

Adds support for booting AArch64 Qemu machines using TF-A + optee +
u-boot. Most of the changes are applicable to any AArch64 qemu target,
and a reference machine called qemuarm64-secureboot has been added that
show how to enable support for it.

Signed-off-by: Joshua Watt <JPEWhacker@gmail.com>
Reviewed-by: Denys Dmytriyenko <denys@ti.com>
Signed-off-by: Jon Mason <jon.mason@arm.com>