boards/arm: stm32l562e_dk: enabling watchdog support in device tree

This commit enables iwdg support for stm32l562e_dk platform in
device tree. This has been tested on stm32l562e_dk platform with
tests/drivers/watchdog/wdt_basic_api and samples/drivers/watchdog.

Signed-off-by: Krishna Mohan Dani <krishnamohan.d@hcl.com>