imx8: Replace PLAT_IMX8* with automatic PLAT_imx8*

Platform defines are already provided by the build system so let's not
duplicate them.

Change-Id: Icf1ea76c3c3213e27b447c95e2b22b961fa7693e
Signed-off-by: Leonard Crestez <leonard.crestez@nxp.com>
6 files changed