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: tickless generic timer implementation

Update the generic timer to implement the MCS kernels tickless timer
driver API and update all platforms that use the arm generic timers:

- bcm2837
- exynos5
- hikey
- imx7
- odroidc2
- tk1
- tx1
- tx2
- zynq7000

Also move the generic timer constants to machine.h to avoid a circular
dependency.

mcs: provide tickless api for arm timers

This does not implement the timers for any platforms, but
provides the generic arm arch, and aarch32/aarch64 infrastructure for
tickless timer drivers.

Combine common arm generic timer code

Previously both aarch32 and aarch64 duplicated common
generic timer code. Unify them in include/arch/arm/machine/timer.h

Initialise gpt_cnt_tval to squash compiler warning

Make TIMER_RELOAD check 64-bit compatable

This check checks the TIMER_RELOAD fits in the wordsize.
Use UINTPTR_MAX instead of 0xffffffff so the check can be moved
to mode agnostic code.

ARM generic timers: use aarch agnostic macros

Make clock check a warning only instead of a hard fault

This clock value isn't set consistently on some platforms due to software
rounding, so halting here tends to happen a lot. Instead print a warning
and continue on.

For example using a default Xilinx platform with a 33.333... MHz oscialltor
and a default bootloader the clock value will be somewhere in the range
99999000-100000000, which for most practical purposes is the same thing.

Since this register is written by software and doesn't have the precision to
fully represent common hardware oscillator frequencies, this check seems overly
strict anyway.

Fix licenses (the rest)

Warn in arm generic timer if reload value is too high

s/D61/DATA61/ in license headers for consistency

ARM: Move timer.h to mode specific folder