watchdog: dw_wdt: No need for a spinlock

Right now the dw_wdt uses a spinlock to protect dw_wdt_open().  The
problem is that while holding the spinlock we call:
-> dw_wdt_set_top()
   -> dw_wdt_top_in_seconds()
      -> clk_get_rate()
         -> clk_prepare_lock()
            -> mutex_lock()

Locking a mutex while holding a spinlock is not allowed and leads to
warnings like "BUG: spinlock wrong CPU on CPU#1", among other
problems.

There's no reason to use a spinlock.  Only dw_wdt_open() was protected
and the test_and_set_bit() at the start of that function protects us
anyway.

Signed-off-by: Doug Anderson <dianders@chromium.org>
Tested-by: Jisheng Zhang <jszhang@marvell.com>
Reviewed-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>