feat(imx8mp): enable BL32 fdt overlay support on imx8mp

Allow OP-TEE to generate a device-tree overlay binary
that will be applied by u-boot on the regular dtb.

Signed-off-by: Jacky Bai <ping.bai@nxp.com>
Change-Id: Idfd268cdd8b7ba321f8e1b9b85c2bba7ffdeddf0