power-domain: Return 0 if ops unimplemented and remove empty functions

In case the ops is not implemented, return 0 in the core right away.
This is better than having multiple copies of functions which just
return 0 in each power domain driver. Drop all those empty functions.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Patrick Delaunay <patrick.delaunay@foss.st.com>
Cc: Simon Glass <sjg@chromium.org>