Revert "ARM: dts: am437x-sk-evm: Correct tps65218 irq type"

This reverts commit 561f9bcf.

While the correct IRQ level fixed the WARN_ON(), but prevented the probe
of tps65218 as GIC_SPI does not support LEVEL_LOW (?)

Use LEVEL_HIGH as it looks to be the accurate one since the INTn of TPS is
connected to NMIn of the SoC.

Fixes: 561f9bcf ("ARM: dts: am437x-sk-evm: Correct tps65218 irq type")
Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>