ARM: dts: am437x-gp-evm: Correct tps65218 irq type

According to the documentation the interrupt line is active low.

The patch will silence the warning from gic_irq_domain_translate():
"Make it clear that broken DTs are... broken"

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Reviewed-by: Keerthy <j-keerthy@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>