power: regulator: scmi: support SCMI multi-channel

Update SCMI regulator controller driver to get its assigned SCMI channel
during initialization. This change allows SCMI voltage domain protocol
to use a dedicated channel when defined in the DT. The reference is
saved in SCMI regulator controller driver private data.

Cc: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Etienne Carriere <etienne.carriere@linaro.org>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>