arm64: dts: arm: drop unused interrupt-names in MHU

arm64: dts: arm: drop unused interrupt-names in MHU

The arm,mhu bindings and driver do not define interrupt-names, so drop
the property to fix warnings:

  arch/arm64/boot/dts/arm/juno-r2.dt.yaml: mhu@2b1f0000: 'interrupt-names' does not match any of the regexes: 'pinctrl-[0-9]+'

Link: https://lore.kernel.org/r/20210820081733.83976-3-krzysztof.kozlowski@canonical.com
Acked-by: Liviu Dudau <liviu.dudau@arm.com>
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>