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