drivers: interrupt_controller: intc_nxp_irqsteer: fix system INTID

Current system INTID calcualtion only worked for SoCs whose extended
interrupts started from IRQ 0.
Otherwise, FSL_FEATURE_IRQSTEER_IRQ_START_INDEX should be added for
system INTID.

Signed-off-by: Yangbo Lu <yangbo.lu@nxp.com>