plat/arm/scmi: introduce plat_css_get_scmi_info API

The default values of 'plat_css_scmi_plat_info' is not applicable for
all the platforms. There should be a provision to let platform code to
register a platform specific instance of scmi_channel_plat_info_t.

Add a new API 'plat_css_get_scmi_info' which lets the platform to
register a platform specific instance of scmi_channel_plat_info_t and
remove the default values.

In addition to this, the existing 'plat_css_scmi_plat_info' structure
is removed from the common code and instantiated for the platforms that
need it. This allows for a consistent provisioning of the SCMI channel
information across all the existing and upcoming platforms.

Change-Id: I4fb65d7f2f165b78697b4677f1e8d81edebeac06
Signed-off-by: Chandni Cherukuri <chandni.cherukuri@arm.com>