convert license headers to SPDX

Includes license review of 3rd-party files, adding missing upstream
headers, and moving the UNSW OZPLB license into its own file rather
than replicating it in the header.

elfloader: use driver model for SMP on ARM

Previously, each platform had its own implementation of init_cpus().
This is particularly cumbersome in the case of aarch64, where all
platforms which support SMP use PSCI to do so. This patch moves
SMP on ARM to be implemented using the driver model.

It introduces a new driver type, DRIVER_SMP. This driver
has a single method, `cpu_on`, which, when given an
`elfloader_cpu` struct, turns on the CPU corresponding to that struct,
launching it at the given entry point with the provided stack.

This patch also makes the aarch64 and aarch32 CPU bringup logic
much more similar. Both architectures now wait for CPUs to come up
before starting the next.

Note that the i.MX7 is left as-is, using the old mechanism, due to a
lack of hardware to test on.