init: drop device union from struct init_entry

Such union is rather redundant, considering a simple const cast can be
done when initializing the init entry. Note that the init_entry does not
need to be touched now that struct device stores the init call. It is
merely an init entry sorted by linker scripts, so we can intertwine
devices and SYS_INIT.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>