power: Add a POWER config

power: Add a POWER config

At present we have SPL_POWER but not piain POWER. This works because
there is a special build rule in Makefile that always includes the
drivers/power directory.

It is better to have all driver directories included by drivers/Makefile
and there is already a rule in there for this purpose. It just needs a
Kconfig for U-Boot proper, so add one.

Update the pmic.h header file so that it defines the old pmic struct
always, when driver model is not in use. That will avoid build errors
for boards which enable POWER but not DM_PMIC.

Enable this option always. That seems strange at first sight, but it
actually but mimics the current Makefile behaviour. Once we can drop the
old PMICs it should be easy enough to rename DM_PMIC to POWWER, or
something similar.

Signed-off-by: Simon Glass <sjg@chromium.org>