clk: qcom: gdsc: Add support for hierarchical power domains

Some qcom SoCs' can have hierarchical power domains. Let the gdsc structs
specify the parents (if any) and the driver add genpd subdomains for them.

Signed-off-by: Rajendra Nayak <rnayak@codeaurora.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>