imx8mp_evk: spl: use spl_early_init

Use spl_early_init to replace spl_init, spl_init will be invoked
in board_init_r, we only need use spl_early_init to setup malloc
and scan early dt.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Fabio Estevam <festevam@gmail.com>