arm64: dts: imx8mq-librem5: Add interrupt-names to ti,tps6598x

The ti,tps6598x binding requires interrupt-names property.  The driver
does not really use it but the hardware could have more interrupt lines
connected.  This fixes dtbs_check warning:

  arch/arm64/boot/dts/freescale/imx8mq-librem5-r2.dt.yaml: usb-pd@3f: 'interrupt-names' is a required property

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>