core: Enable DM by default

There are no longer any platforms which do not enable DM, move this to a
def_bool y and remove the check in the Makefile.

Cc: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/core/Kconfig b/drivers/core/Kconfig
index c9bf5de..0dc442b 100644
--- a/drivers/core/Kconfig
+++ b/drivers/core/Kconfig
@@ -1,7 +1,7 @@
 menu "Generic Driver Options"
 
 config DM
-	bool "Enable Driver Model"
+	def_bool y
 	help
 	  This config option enables Driver Model. This brings in the core
 	  support, including scanning of platform data on start-up. If