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: Implement timer functions for RISC-V

Introduce the functions needed to read the time and set the timer for
MCS in RISC-V.

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.

arm: remove plat/machine/timer.h

plat/machine/timer.h is unused on arm, so remove it and fix up the
includes.