plat/arm/sgi: Use platform specific functions to get platform ids

Add two new functions 'plat_arm_sgi_get_platform_id' and
'plat_arm_sgi_get_config_id' which will be implemented by all the
SGI platforms. These functions can be used to determine the part
number and configuration id of the SGI platforms.

In BL2, these functions are used to populate the 'system-id' node.
In BL31, these functions are used to populate the 'sgi_plat_info_t'
structure with the part number and configuration id of the platform.

Change-Id: I3bacda933527724a3b4074ad4ed5b53a81ea4689
Signed-off-by: Chandni Cherukuri <chandni.cherukuri@arm.com>
diff --git a/plat/arm/css/sgi/include/sgi_variant.h b/plat/arm/css/sgi/include/sgi_variant.h
index dea580b..56dc334 100644
--- a/plat/arm/css/sgi/include/sgi_variant.h
+++ b/plat/arm/css/sgi/include/sgi_variant.h
@@ -21,4 +21,10 @@
 
 extern sgi_platform_info_t sgi_plat_info;
 
+/* returns the part number of the platform*/
+unsigned int plat_arm_sgi_get_platform_id(void);
+
+/* returns the configuration id of the platform */
+unsigned int plat_arm_sgi_get_config_id(void);
+
 #endif /* SGI_VARIANT_H */