clk: qcom: add support for power domains uclass

clk: qcom: add support for power domains uclass

Now sub-drivers for particular SoCs can register them as power domain
drivers. This is needed for upcoming SM8150 support, because it needs
to power up the Ethernet module.

Signed-off-by: Volodymyr Babchuk <volodymyr_babchuk@epam.com>
Reviewed-by: Sumit Garg <sumit.garg@linaro.org>
Reviewed-by: Caleb Connolly <caleb.connolly@linaro.org>
[caleb: make ARCH_SNAPDRAGON select POWER_DOMAIN]
Signed-off-by: Caleb Connolly <caleb.connolly@linaro.org>