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>