dt-bindings: watchdog: dw-wdt: Support devices with asynch clocks

DW Watchdog IP core can be synthesised with asynchronous timer/APB
clocks support (WDT_ASYNC_CLK_MODE_ENABLE == 1). In this case
separate clock signals are supposed to be used to feed watchdog timer
and APB interface of the device. Let's update the DW Watchdog DT node
schema so it would support the optional APB3 bus clock specified along
with the mandatory watchdog timer reference clock.

Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Cc: Alexey Malahov <Alexey.Malahov@baikalelectronics.ru>
Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: linux-mips@vger.kernel.org
Link: https://lore.kernel.org/r/20200530073557.22661-3-Sergey.Semin@baikalelectronics.ru


Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>