PM: domains: Drop redundant header for genpd

The "power.h" is no longer needed by genpd, so let's simply drop the
include of it.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Acked-by: Rafael J. Wysocki <rafael@kernel.org>
Link: https://lore.kernel.org/r/20231213113245.29075-1-ulf.hansson@linaro.org