rename symbol: CONFIG_TEGRA -> CONFIG_ARCH_TEGRA

Have this symbol follow the pattern of all other such symbols.

Signed-off-by: Trevor Woerner <twoerner@gmail.com>
diff --git a/include/serial.h b/include/serial.h
index 54b21a0..c590637 100644
--- a/include/serial.h
+++ b/include/serial.h
@@ -28,7 +28,7 @@
 
 #if	defined(CONFIG_MPC83xx) || defined(CONFIG_MPC85xx) || \
 	defined(CONFIG_MPC86xx) || \
-	defined(CONFIG_TEGRA) || defined(CONFIG_SYS_COREBOOT) || \
+	defined(CONFIG_ARCH_TEGRA) || defined(CONFIG_SYS_COREBOOT) || \
 	defined(CONFIG_MICROBLAZE)
 extern struct serial_device serial0_device;
 extern struct serial_device serial1_device;