watchdog: dw_wdt: get reset lines from dt

The dw_wdt has an external reset line, that can keep the device in reset
and therefore rendering it useless and also is the only way of stopping
the watchdog once it was started.

Get the reset lines for this core from the devicetree. As these lines are
optional, use devm_reset_control_get_optional_shared. If the reset line
is not specified in the devicetree, the reset framework will just skip
deasserting and continue.
This way all users of the driver will continue to function without
any harm, even if the reset line is not specified in the devicetree.

Signed-off-by: Steffen Trumtrar <s.trumtrar@pengutronix.de>
Cc: linux-watchdog@vger.kernel.org
Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>