powercap/drivers/dtpm: Add CPU DT initialization support

Based on the previous DT changes in the core code, use the 'setup'
callback to initialize the CPU DTPM backend.

Code is reorganized to stick to the DTPM table description. No
functional changes.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Link: https://lore.kernel.org/r/20220128163537.212248-4-daniel.lezcano@linaro.org