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.

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.

RISC-V: Refactor IRQ handling to use PLIC

Previously we only handled IRQs generated by the BBL running in machine
mode via changes to supervisor interrupt pending (SIP) bits. Now that we
need to support shared global interrupts through the PLIC we need to
modify the way interrupts are processed.

We additionally remove many DONT_TRANSLATE annotations on functions that
the CParser is able to parse.

Also authored by: Siwei Zhuang <siwei.zhuang@data61.csiro.au>

RISC-V: Add PLIC interface and HiFive driver

We assume the PLIC is currently the only global interrupt controller
that RISC-V platforms are going to use. Each platform may have a
different programmers model for interracting with the hardware
controller. We provide a common interface for PLIC drivers to implement
that the kernel will use to manage IRQs.

Also authored by: Siwei Zhuang <siwei.zhuang@data61.csiro.au>

RISC-V: Add kernel device functionalty

- Create device untypeds for platform devices that can be passed to user
space.
- Create kernel device mapping for devices required by the kernel.
Note that the current kernel device mapping is simply a 1GiB of page
physical memory as that contains all devices for all RISC-V platforms
that are currently supported.

spike: Refactor into clearer subplatforms

For now we keep the RISC-V platforms under the spike kernel platform
name. Splitting the different platform definitions into different header
files will help maintainability as we add different device layouts and
kernel device drivers.