power: Rename CONFIG_POWER to CONFIG_POWER_LEGACY

power: Rename CONFIG_POWER to CONFIG_POWER_LEGACY

This option is used in pre-driver model code and much of it has never
been converted to driver model.

We want to add a new option to enable power support, so we can use a
simple rule in the Makefile. Rename this one, which is really about
a particular implementation of power.

Also update the pmic.h header file so it either includes the legacy
API or the driver model one.

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