arm/imx21: add kbuild support for the Freescale i.MX21

* adds Kconfig variables
* specifies different physical address for i.MX21 because of the
  different memory layouts
* disables support for UART5/UART6 in the i.MX serial driver
  (the i.MX21 doesn't have those modules)

Based on code from "Martin Fuzzey" <mfuzzey@gmail.com>

Signed-off-by: Holger Schurig <hs4233@mail.mn-solutions.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
diff --git a/arch/arm/mach-mx2/serial.c b/arch/arm/mach-mx2/serial.c
index 16debc2..b9e66fb 100644
--- a/arch/arm/mach-mx2/serial.c
+++ b/arch/arm/mach-mx2/serial.c
@@ -99,6 +99,7 @@
 	.num_resources = ARRAY_SIZE(uart3),
 };
 
+#ifdef CONFIG_MACH_MX27
 static struct resource uart4[] = {
 	{
 		.start = UART5_BASE_ADDR,
@@ -136,3 +137,4 @@
 	.resource = uart5,
 	.num_resources = ARRAY_SIZE(uart5),
 };
+#endif