firmware: scmi: prepare scmi uclass API to multi-channel

Changes SCMI driver API function devm_scmi_process_msg() to add
an SCMI channel reference argument for when SCMI agent supports
SCMI protocol specific channels. First argument of devm_scmi_process_msg()
is also change to point to the caller SCMI protocol device rather
than its parent device (the SCMI agent device).

The argument is a pointer to opaque struct scmi_channel known from
the SCMI transport drivers. It is currently unused and caller a pass
NULL value. A later change will enable such support once SCMI protocol
drivers have means to get the channel reference during initialization.

Cc: Lukasz Majewski <lukma@denx.de>
Cc: Sean Anderson <seanga2@gmail.com>
Cc: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Etienne Carriere <etienne.carriere@linaro.org>