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: generic_timer -> drivers/timer/arm_generic

- move generic_timer.h to drivers/timer/arm_generic.h
- update all configs that use it.

exynos: use declare_default_headers

- remove unused header files replaced by declare_default_headers
- move timer.h to drivers/timer/exynos5422-mct.h
- move mct.h to drivers/timer/mct.h

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.