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.

elfloader: pick serial driver at runtime

On ARM, each serial driver now has an ELFLOADER_DRIVER(...), which
contains information about the compatible strings the driver
matches, the driver type and an "init" function for the driver to do
setup.

hardware_gen is now used by the ELF loader to generate a list of
interesting devices at compile-time, which go in a header file
"devices_gen.h".

The ELFLOADER_DRIVERs are put into a special "_driver_list" section
by the linker, and accessed as an array via the special
__start__driver_list and __stop__driver_list linker symbols.

At boot, the ELF loader calls initialise_devices(), which does the
following:
for each device in devices_gen.h
for each driver in elfloader_drivers array
if (device compat matches driver compat)
call driver->init(device, driver)
end if
end for
end for

For serial drivers, the serial driver calls uart_set_out with
the elfloader_device that should be the output for the elfloader
from the driver->init function.

There is a 'common' plat_console_putchar implementation which uses
the dynamically picked uart output. This eliminates the need
for a lot of the platform-specific code we have.

This implementation could one day be replaced with a in-ELF loader
device tree. This could also (possibly) be used to handle dynamically
picking SMP core bringup mechanisms.

As all RISC-V platforms use SBI to provide a serial port, RISCV is
only modified to register the SBI putc implementation as the fputc
implementatin.