clk: scmi: support SCMI multi-channel

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

Cc: Lukasz Majewski <lukma@denx.de>
Cc: Sean Anderson <seanga2@gmail.com>
Signed-off-by: Etienne Carriere <etienne.carriere@linaro.org>