Add plugin.S for plugin boot initialization on mx6dq/dl arm2 board.
Need to set "CONFIG_USE_IMXIMG_PLUGIN" for this feature.

New build configurations are added for the plugin:
mx6dlarm2_lpddr2_plugin_defconfig
mx6dlarm2_plugin_defconfig
mx6qarm2_lpddr2_plugin_defconfig
configs/mx6qarm2_plugin_defconfig

Signed-off-by: Ye Li <ye.li@nxp.com>