spl: imx: use trampoline buffer to load images to secure region

spl: imx: use trampoline buffer to load images to secure region

When SPL loading image to secure region, for example, ATF and tee to
DDR secure region. Because the USDHC controller is non-secure master,
it can't access this region and will cause loading issue.

So use a trampoline buffer in non-secure region, then use CPU to copy the
image from trampoline buffer to destination secure region.

Signed-off-by: Ye Li <ye.li@nxp.com>
Signed-off-by: Alice Guo <alice.guo@nxp.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>