board: fsl: lx2160a: free up arch_misc_init

Currently LX2 board code uses arch_misc_init to set up the board mux on
RDB and QDS.  Move this code to misc_init_r.  This is consistent with LS
gen 2 and T series SoCs/boards.

Signed-off-by: Alex Marginean <alexandru.marginean@nxp.com>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>