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.

arm,SMP: Refactor irq_t structure for smp

Explicitly create a struct definition for irq_t on SMP Arm
configurations. This makes it a lot harder to mistakenly use the wrong
irq encoding when moving an irq between a cnode index and hardware irq
number / core. A couple areas where this was being handled incorrectly
was fixed as part of the refactor. When performing an ipi for masking
PPI interrupts, the idx encoding is used as it fits into a single word.

Document internal machine/interrupt.h interface

Every platform has to implement a standard set of interrupt interfaces
that the kernel uses to interract with a machine's interrupt controller.
Providing a single header file for each of these functions provides a
single location to document their behavior.