configs: am335x_evm: Enable SPL_DM

Enable SPL_DM on all AM335x based TI platforms.

http://patchwork.ozlabs.org/patch/751300/
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
diff --git a/include/configs/ti_am335x_common.h b/include/configs/ti_am335x_common.h
index c0e4336..e4c3c80 100644
--- a/include/configs/ti_am335x_common.h
+++ b/include/configs/ti_am335x_common.h
@@ -22,8 +22,10 @@
 /* NS16550 Configuration */
 #ifdef CONFIG_SPL_BUILD
 #define CONFIG_SYS_NS16550_SERIAL
+#ifndef CONFIG_DM_SERIAL
 #define CONFIG_SYS_NS16550_REG_SIZE	(-4)
 #endif
+#endif
 #define CONFIG_SYS_NS16550_CLK		48000000
 
 #ifndef CONFIG_SPL_BUILD