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.

mcs: add mcs for imx8mmq

Move TICKS_PER_MS out of ifdef guard,
so that the gicv3 can find it.
Turn of buildsystem check for mcs.
Add magic constants to config.

mcs: provide tickless api for arm timers

This does not implement the timers for any platforms, but
provides the generic arm arch, and aarch32/aarch64 infrastructure for
tickless timer drivers.

Move arm generic timer to own header

Only platforms that use it should include the code

Move definition of initTimer to be arm specific

initTimer does not exist on x86

Combine common arm generic timer code

Previously both aarch32 and aarch64 duplicated common
generic timer code. Unify them in include/arch/arm/machine/timer.h

Remove many DONT_TRANSLATE markers.

The vast majority of the DONT_TRANSLATE markers in the kernel are used
to hide __asm__ statements and builtin functions
(e.g. __builtin_unreachable ()) from the C-to-Isabelle parser.

The parser now supports underscore identifiers and many __asm__ statements,
and the builtin functions are prototyped, meaning the vast majority of the
DONT_TRANSLATE markers can be dropped. The remaining markers cover functions
that must be treated specially.

Replace #pragma once with include guards

SELFOUR-612: one generic timer impl

This commit unifies 3 implementations of the arm
generic timer and cleans up existing code.