init: Make entry init-function less and introduce service objects

Since the addition of deinit operation in device, init and deinit have
been within each device, rendering their init entry's init function
useless.

In order to save ROM space, let's remove the init function from
init entry altogether, and introduce a new object called "service"
which owns an init function to go along with SYS_INIT/SYS_INIT_NAMED.

Signed-off-by: Tomasz Bursztyka <tobu@bang-olufsen.dk>