Flush posted write to IRQSTATUS register in GPIO IRQ handler.
This eliminates the below error for all peripherals that use GPIO interrupts.

<4>Spurious irq 95: 0xffffffdf, please flush posted write for irq 31
Signed-off-by: Roger Quadros <ext-roger.quadros@nokia.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>