watchdog: designware: make reset really optional

Checking for DM_RESET is not enough since not all watchdog
implementations use a reset lane. Such is the case for Rockchip
implementation for example. Since reset_assert_bulk will only succeed if
the resets property exists in the watchdog DT node, it needs to be
called only if a reset property is present.

This adds a condition on the resets property presence in the watchdog DT
node before assuming a reset lane needs to be fetched with
reset_assert_bulk, by calling ofnode_read_prop.

Cc: Quentin Schulz <foss+uboot@0leil.net>
Reviewed-by: Stefan Roese <sr@denx.de>
Signed-off-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>