MLK-22105-3 imx8/imx8m: spl: Move bss clean up before arch_cpu_init

MLK-22105-3 imx8/imx8m: spl: Move bss clean up before arch_cpu_init

Since rng_init is used arch_cpu_init, we have to clean up BSS section
before it.
Also remove the unnecessary memset to global data, because
board_init_f_init_reserve already memset it. If we memset it in board_init_f,
the gd->malloc_base is reset to 0 and will cause early malloc problem
when CONFIG_MALLOC_F_ADDR is not set.

Signed-off-by: Ye Li <ye.li@nxp.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
(cherry picked from commit c36ae6a623b075192b20d93f9e3fe966961d86f4)