misc: scu_api: Add SCFW API to get the index of boot container set

Add SCFW API sc_misc_get_boot_container to get current boot container
set index.
The index value returns 1 for primary container set, 2 for secondary
container set.

Signed-off-by: Ye Li <ye.li@nxp.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>