watchdog: dw_wdt: remove build dependencies

There is nothing ARM specific in this driver, and we intend to use it on the
Xtensa architecture. Also, clk.h now includes stubs for !CONFIG_HAVE_CLK, so
the driver should build anyway.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Acked-by: Jamie Iles <jamie@jamieiles.com>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>