replacing all ifndef with pargma once

All the kernel header files now use pargma once rather than the ifndef,
as the pre-processed C files do not change while header files
are protected with pargma once. This will also solve any naming issues
caused by ifndef.

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.

Fix licenses (the rest)

s/D61/DATA61/ in license headers for consistency

SMP: Abstract architecture/platform independent parts of smp.h and ipi.h

This commit is a re-arrangement of SMP directory structure to make it
easier for other architectures/platforms (in general) and ARM (in
particular) to add SMP support.

* new include/smp directory to act as a centralised container of
"shared" architecture-independent SMP headers.
This makes it clearer what's needed for other architecture/platform to
support SMP.
* Each platform can define its own unique ipi.[h|c] that make sense,
since ipi implementation is SoC/platform dependent.

ia32: Use wrappers for remote ops

SELFOUR-634: support for TLB and cache management