common: spl: atf: support booting bl32 image

Trusted-Firmware can also initialize a secure payload to use as a trusted
execution environment. In general for the arm64 case this is provided as
separate image and uboot is supposed to also place it in a predetermined
location in memory and add the necessary parameters to the ATF boot params.

So add the possibility to get this tee payload from the provided FIT image
and setup things as necessary.

Tested on a Rockchip PX30 with mainline TF-A, mainline OP-Tee (with pending
PX30 support) and mainline 5.4-rc1 Linux kernel.

Signed-off-by: Joseph Chen <chenjh@rock-chips.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>