arm64: dts: imx8mq-librem5: Drop interrupt-names in PMIC

The 'interrupt-names' property is not described in dtschema, not used by
the driver and does not really make sense as its value is simple 'irq'.
Drop it to fix dtbs_check warnings like:

  arch/arm64/boot/dts/freescale/imx8mq-librem5-devkit.dt.yaml:
    pmic@4b: 'interrupt-names' does not match any of the regexes: 'pinctrl-[0-9]+'

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