arc: Kconfig: Add missing DM dependency

ARC is selecting TIMER which depends on DM but DM is not selected and
doesn't need to be enabled. Fix it by selecting DM for ARC architecture.

Kconfig is showing this missing dependency by:
WARNING: unmet direct dependencies detected for TIMER
  Depends on [n]: DM [=n]
  Selected by [y]:
  - ARC [=y] && <choice>

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

arch/Kconfig

@@ -12,6 +12,7 @@ config ARC
 	bool "ARC architecture"
 	select ARC_TIMER
 	select CLK
+	select DM
 	select HAVE_PRIVATE_LIBGCC
 	select SUPPORT_OF_CONTROL
 	select TIMER