[ARM] MX1: Add missing selection of ARM920T

The MX1 only has one possible CPU type, ARM920T. Select it in
Kconfig.
Signed-off-by: NSascha Hauer <s.hauer@pengutronix.de>

arch/arm/plat-mxc/Kconfig

@@ -8,6 +8,7 @@ choice
 
 config ARCH_MX1
 	bool "MX1-based"
+	select CPU_ARM920T
 	help
 	  This enables support for systems based on the Freescale i.MX1 family