trivial: fix header files

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

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: tickless driver for x86

Add a tickless timer driver for x86. The driver defaults to using
TSC_DEADLINE mode, but falls back to the apic if that feature is not
available.

arm: remove plat/machine/timer.h

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

Fix licenses (the rest)

s/D61/DATA61/ in license headers for consistency

x86: export tsc freq in bootinfo

We read the frequency from the platform info MSR and export it to the
user for accurate timing (for platforms that support platform into)