arm64: dts: juno: replace '_' with '-' in node names

The latest DTC throws warnings for character '_' in the node names.

Warning (node_name_chars_strict): /thermal-zones/big_cluster: Character '_' not recommended in node name
Warning (node_name_chars_strict): /thermal-zones/little_cluster: Character '_' not recommended in node name
Warning (node_name_chars_strict): /smb@8000000/motherboard/gpio_keys: Character '_' not recommended in node name
Warning (node_name_chars_strict): /pmu_a57: Character '_' not recommended in node name
Warning (node_name_chars_strict): /pmu_a53: Character '_' not recommended in node name

The general recommendation is to use character '-' for all the node names.
This patch fixes the warnings following the recommendation.

Acked-by: Liviu Dudau <liviu.dudau@arm.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>