arm64: dts: stratix10: move ARM timer out of SoC node

The ARM timer is usually considered not part of SoC node, just like
other ARM designed blocks (PMU, PSCI).  This fixes dtbs_check warning:

  arch/arm64/boot/dts/altera/socfpga_stratix10_socdk.dt.yaml: soc: timer:
    {'compatible': ['arm,armv8-timer'], 'interrupts': [[1, 13, 3848], [1, 14, 3848], [1, 11, 3848], [1, 10, 3848]]} should not be valid under {'type': 'object'}
    From schema: dtschema/schemas/simple-bus.yaml

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Signed-off-by: Dinh Nguyen <dinguyen@kernel.org>