drivers: watchdog: rts5912: use portable IRQ pending API

Replace direct NVIC_*PendingIRQ()/NVIC_EnableIRQ()/NVIC_DisableIRQ()
calls with k_irq_clear_pending()/k_irq_set_pending()/k_irq_is_pending() and
irq_enable()/irq_disable(), dropping the driver's dependency on
cmsis_core.h where nothing else needed it.

Assisted-by: Claude:claude-opus-5
Signed-off-by: Anas Nashif <anas.nashif@intel.com>

Author: nashif

drivers/watchdog/wdt_rts5912.c

@@ -225,7 +225,7 @@ static int wdt_rts5912_init(const struct device *dev)
 	wdt_reg->DIV = cfg->div;
 
 	wdt_reg->CTRL |= WDT_CTRL_CLRRSTFLAG;
-	NVIC_ClearPendingIRQ(DT_INST_IRQN(0));
+	k_irq_clear_pending(DT_INST_IRQN(0));
 
 	IRQ_CONNECT(DT_INST_IRQN(0), 0, wdt_rts5912_isr, DEVICE_DT_INST_GET(0), 0);
 	irq_enable(DT_INST_IRQN(0));