power: Refactor Makefile rules

Move the power/ rules into drivers/power to avoid clutter in the Makefile
and drivers/Makefile files.

We must select SPL_POWER if SPL_POWER_DOMAIN is used, since the two are
currently independent and boards do not necessarily enable SPL_POWER.

Add a TPL_POWER as well, as that is used by one board.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>