mx7ulp: soc: s_init should only be executed once

On SPL enabled systems, the current s_init code (wdog, clock and ldo
init) is executed twice (by SPL and u-boot). This is not necessary and
might lead to boot issues (ie, starting PMC1 when it is already running).

Signed-off-by: Jorge Ramirez-Ortiz <jorge@foundries.io>
Reviewed-by: Fabio Estevam <festevam@gmail.com>