drivers: timer: 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/timer/realtek_rts5912_rtmr.c

@@ -204,7 +204,7 @@ static int sys_clock_driver_init(void)
 {
 	/* Enable RTMR clock power */
 	RTMR_REG->INTSTS = RTOSTMR_INTSTS_STS_Msk;
-	NVIC_ClearPendingIRQ(DT_INST_IRQN(0));
+	k_irq_clear_pending(DT_INST_IRQN(0));
 
 	SYSTEM_Type *sys_reg = RTS5912_SCCON_REG_BASE;