Convert CONFIG_MX7D to Kconfig

This converts the following to Kconfig:
   CONFIG_MX7D

Cc: Oleksandr Suvorov <oleksandr.suvorov@toradex.com>
Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/arch/arm/mach-imx/mx7/Kconfig b/arch/arm/mach-imx/mx7/Kconfig
index adedc01..059e658 100644
--- a/arch/arm/mach-imx/mx7/Kconfig
+++ b/arch/arm/mach-imx/mx7/Kconfig
@@ -91,6 +91,7 @@
 	select DM
 	select DM_SERIAL
 	select DM_THERMAL
+	select MX7D
 	imply CMD_DM
 
 endchoice