imx8: cpu: restrict checking ROM passover info for revA

imx8: cpu: restrict checking ROM passover info for revA

Passover info only for revA.

move get_cpu_rev out of CONFIG_CPU to avoid build failure when using
get_cpu_rev in SPL.
Add a CONFIG_SPL_BUILD for passover usage, no need to execute it again
in normal U-Boot stage. Also if still checking passover info in normal
U-Boot stage, need to make the passover code executed after
arch_cpu_init_dm.
So to make it easy and clean, only execute the code for SPL stage.

Signed-off-by: Peng Fan <peng.fan@nxp.com>