ARM: dts: vexpress: replace '_' with '-' in node names

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

Warning (node_name_chars_strict): /pmu_a15: Character '_' not recommended in node name
Warning (node_name_chars_strict): /pmu_a7: 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>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>