Convert license tags to SPDX identifiers

This commit also converts our own copyright headers to directly use
SPDX, but leaves all other copyright header intact, only adding the
SPDX ident. As far as possible this commit also merges multiple
Data61 copyright statements/headers into one for consistency.

hardware_gen: add elfloader CPUs output

outputs all CPUs described in the DT to the elfloader
header and also includes any devices in the seL4,elfoader-devices
property.

hardware_gen: Always specify kernel devices

The kernel device IRQs and Frame mappings generated by this script will
only come from nodes specified in the seL4,kernel-devices property of
the chosen node. Previously these devices were inferred by the script
but this led to false matching and didn't support easily overriding
which devices to match under different configurations or across
different platforms.

Explicitly specifying which devices from the device tree will be used in
the kernel makes it easier to check which devices the kernel is actually
using and makes it easier to change on a per platform or per
configuration basis.

exynos4, exynos5: add DTS overlay comments

This explains why the DTS overlays intentionally throw away part of
the kernel memory window.

exynos4, exynos5, sabrelite: clamp memory to fit in kernel window

This is a hack that allows user-space tools like the capDL static
allocator to work. Otherwise, the allocator (currently unaware of the
kernel window) would think that there is a 28-bit untyped at pptr
0xf0000000, and would generate untyped derivations that are not
available at runtime.

Note that we clamp to 0xff000000 on exynos4 and exynos5, *not*
0xfff00000 (PPTR_TOP), to avoid the ASID PD hole at
0xff000000..0xff200000. The affected platforms are ARM VM platforms;
VM images are often larger than 13MiB and would overlap the ASID hole
if loaded from PPTR_TOP. init_freemem fails if this overlap exists.

exynos5: keep the first 1GiB of RAM for VMs

This moves the kernel back up to 0x60000000 as it was before
the HW headers were autogenerated, as this behaviour is depended on by
some user-level projects.

dts: add seL4 overlays

These contain various fixes that the Linux DTSs need
to generate usable output for seL4.