arm64: dts: layerscape: fix thermal node names length

Linux kernel expects thermal zone node names to be maximum of 19
characters (see THERMAL_NAME_LENGTH, including terminating NUL byte) and
bindings/dtbs_check points that:

  fsl-ls2088a-rdb.dtb: thermal-zones: 'core-cluster1-thermal', 'core-cluster2-thermal', 'core-cluster3-thermal', 'core-cluster4-thermal'
    do not match any of the regexes: '^[a-zA-Z][a-zA-Z0-9\\-]{1,10}-thermal$', 'pinctrl-[0-9]+'

Name longer than 19 characters leads to driver probe errors when
registering such thermal zone.

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