The QEMU platform port scans its device tree to advertise PSCI as the
CPU enable method. It does this by scanning *every* node in the DT and
check whether its compatible string starts with "arm,cortex-a". Then it
sets the enable-method to PSCI, if it doesn't already have one.

Other platforms might want to use this functionality as well, so let's
move it out of the QEMU platform directory and make it more robust by
fixing some shortcomings:
- A compatible string starting with a certain prefix is not a good way
to find the CPU nodes. For instance a "arm,cortex-a72-pmu" node will
match as well and is in turn favoured with an enable-method.
- If the DT already has an enable-method, we won't change this to PSCI.

Those two issues will for instance fail on the Raspberry Pi 4 DT.
To fix those problems, we adjust the scanning method:
The DT spec says that all CPU nodes are subnodes of the mandatory
/cpus node, which is a subnode of the root node. Also each CPU node has
to...