dts: arm: nuvoton: add wdt node of numaker m2l31x

[cyliangtw]

This PR is to add watchdog node of numaker m2l31x dtsi and enable watchdog of numaker_m2l31ki board for wdt-basic-api test.

Pass wdt-basic-api test as:

Running TESTSUITE wdt_basic_test_suite
===================================================================
START - test_wdt
Testcase: test_wdt_no_callback
Waiting to restart MCU
Running TESTSUITE wdt_basic_test_suite
===================================================================
START - test_wdt
Testcase: test_wdt_no_callback
Testcase passed
Testcase: test_wdt_callback_1
Waiting to restart MCU
Running TESTSUITE wdt_basic_test_suite
===================================================================
START - test_wdt
Testcase: test_wdt_callback_1
Testcase passed
Testcase: test_wdt_bad_window_max
E: window.max should be non-zero
 PASS - test_wdt in 0.011 seconds
===================================================================
TESTSUITE wdt_basic_test_suite succeeded

------ TESTSUITE SUMMARY START ------

SUITE PASS - 100.00% [wdt_basic_test_suite]: pass = 1, fail = 0, skip = 0, total = 1 duration = 0.011 seconds
 - PASS - [wdt_basic_test_suite.test_wdt] duration = 0.011 seconds

------ TESTSUITE SUMMARY END ------

===================================================================
PROJECT EXECUTION SUCCESSFUL