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.

omap3: implement tickless timer driver

Update the existing omap driver to implement the new tickless api.

arm: remove plat/machine/timer.h

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

style: consistently attach return type

Add attach-return-type to astyle

timer-refactor: Move kernel timers and rename

This change moves all arm platform timers to drivers/timer
and renames them to the compatibility string specified in the dts.
Now the correct timer is included at build time based on the dts,
this follows the same pattern as the serial-refactor.
Have moved the arm generic_timer and priv_timer into the timer
folder and updated cmake configs appropriately.