Cater for preloaded BL33 within plat_get_ns_image_entrypoint()

The PRELOADED_BL33_BASE build option allows to preload a BL33 and bypass its
loading by BL2. In ARM standard platforms, the conditional behaviour of
PRELOADED_BL33_BASE is moved within the implementation of
`plat_get_ns_image_entrypoint()` so that all callers may benefit from this
feature.

Change-Id: Iea060e204ec72f8081087837854535c4e320da4e
diff --git a/plat/arm/common/arm_common.c b/plat/arm/common/arm_common.c
index 33d2b06..03e44f4 100644
--- a/plat/arm/common/arm_common.c
+++ b/plat/arm/common/arm_common.c
@@ -114,7 +114,11 @@
 
 uintptr_t plat_get_ns_image_entrypoint(void)
 {
+#ifdef PRELOADED_BL33_BASE
+	return PRELOADED_BL33_BASE;
+#else
 	return PLAT_ARM_NS_IMAGE_OFFSET;
+#endif
 }
 
 /*******************************************************************************