fix(scmi): change function prototype to fix gcc error

fix(scmi): change function prototype to fix gcc error

Change function prototype of plat_css_get_scmi_info() to fix the GCC
sign conversion error "comparison between signed and unsigned integer
expressions". Changing channel_id type to unsigned int since it can
never be a negative value.

Signed-off-by: Tony K Nadackal <tony.nadackal@arm.com>
Change-Id: I579b21497329db40897c10d86c8fc68e4877f3db