ARM: imx: Change the way nand devices are registered (imx21)

Make use of new mechanism to register a nand device.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>

arch/arm/mach-imx/devices.h

@@ -26,9 +26,6 @@ extern struct platform_device imx2x_uart_device4;
 extern struct platform_device imx2x_uart_device5;
 #endif
 extern struct platform_device mxc_w1_master_device;
-#ifdef CONFIG_MACH_MX21
-extern struct platform_device imx21_nand_device;
-#endif
 #ifdef CONFIG_MACH_MX27
 extern struct platform_device imx27_nand_device;
 #endif