ARM: multi_v7_defconfig: enable DW_WATCHDOG

The watchdog timer on the SoCFPGA platform is the Synopsys Designware watchdog.
Enable CONFIG_DW_WATCHDOG for the driver to get built.

Signed-off-by: Dinh Nguyen <dinguyen@opensource.altera.com>
Tested-by: Kevin Hilman <khilman@baylibre.com>
Signed-off-by: Olof Johansson <olof@lixom.net>