it8xxx2: intc: Look for pending interrupt

The AIVECT (All Interrupt Vector Register) register provides us a quick
way to determine SOC interrupt number. But sometimes we get interrupt
number 0 from the register and caused assertion of unhandled interrupt.
To avoid the assertion, we look for all ISRs (Interrupt Status Register)
and IERs (Interrupt Enable Register) to find the highest priority pending
interrupt and return it to caller.

Signed-off-by: Dino Li <Dino.Li@ite.com.tw>