Fix comment of plat_reset_handler stub

As described in the Porting Guide, plat_reset_handler should
preserve x19 to x29.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

plat/common/aarch64/platform_helpers.S

@@ -98,7 +98,7 @@ endfunc plat_crash_console_putc
 
 	/* -----------------------------------------------------
 	 * Placeholder function which should be redefined by
-	 * each platform. This function should preserve x10.
+	 * each platform. This function should preserve x19 - x29.
 	 * -----------------------------------------------------
 	 */
 func plat_reset_handler