Modify return type of plat_get_ns_image_entrypoint()

This patch modifies the return type of the platform API
`plat_get_ns_image_entrypoint()` from `unsigned long` to
`uintptr_t` in accordance with the coding guidelines.

Change-Id: Icb4510ca98b706aa4d535fe27e203394184fb4ca
diff --git a/include/plat/common/platform.h b/include/plat/common/platform.h
index 5c61f38..6f0a8a0 100644
--- a/include/plat/common/platform.h
+++ b/include/plat/common/platform.h
@@ -60,7 +60,7 @@
 int plat_get_image_source(unsigned int image_id,
 			uintptr_t *dev_handle,
 			uintptr_t *image_spec);
-unsigned long plat_get_ns_image_entrypoint(void);
+uintptr_t plat_get_ns_image_entrypoint(void);
 unsigned int plat_my_core_pos(void);
 int plat_core_pos_by_mpidr(u_register_t mpidr);