risc-v: Implement benchmark timestamp

Implements timestamp function used by benchmark logging utilities.

Signed-off-by: Curtis Millar <curtis.millar@data61.csiro.au>
Signed-off-by: Curtis Millar <curtis@curtism.me>

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.

Create device untypeds at boot for all arches

Currently on x86 device untypeds are generated by passing the entire
address space minus any parts that are reserved by the kernel or that
are "real" memory (e.g. kernel image, physical RAM).

On ARM and RISC-V, device untypeds were generated at compile-time from
a device tree. This patch moves ARM and RISC-V to use the same approach
as x86, and moves the code from x86 into a common location that's
shared between the three architectures.

Co-Authored-By: Anna Lyons <anna@gh.st>

CMake: Generate device headers from DTS for spike

The DTS compilation was arm platforms only. Moving it to the top level
config file, making it available to RISCV platforms. The generated files
are almost identical with minor differences. A new argument(--arch) is
added to the hardware_gen.py for the differences.

[SELFOUR-1156] RISC-V Port

Experimental release that supports both RV32 and RV64