clk: qcom: gdsc: Add support for gdscs with RETENTION power state

Some gdscs can support an intermediate RETENTION state apart from
the ON and OFF states. The RETENTION state can further be configured
to either retain all memory, or just some part of it (periperal or
core).
This patch adds support so gdscs can support the additional RET state
which the client drivers can then choose based on acceptable
latency.

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