armv8: fsl-layerscape: introduce fsl_board_late_init()

The fsl-layerscape already occupies board_late_init(), therefore it is
not possible for a board to have its own board_late_init(). Introduce
fsl_board_late_init() which can be implemented in the board specific
code.

Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>