Add support for %p in tf_printf()

This patch adds support for the `%p` format specifier in tf_printf()
following the example of the printf implementation of the stdlib used
in the trusted firmware.

Fixes ARM-software/tf-issues#292

Change-Id: I0b3230c783f735d3e039be25a9405f00023420da
diff --git a/bl31/runtime_svc.c b/bl31/runtime_svc.c
index 5b7a21c..f011f11 100644
--- a/bl31/runtime_svc.c
+++ b/bl31/runtime_svc.c
@@ -103,8 +103,8 @@
 		 */
 		rc = validate_rt_svc_desc(&rt_svc_descs[index]);
 		if (rc) {
-			ERROR("Invalid runtime service descriptor 0x%lx (%s)\n",
-					(uintptr_t) &rt_svc_descs[index],
+			ERROR("Invalid runtime service descriptor %p (%s)\n",
+					(void *) &rt_svc_descs[index],
 					rt_svc_descs[index].name);
 			goto error;
 		}