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.

cortex-a9: tickless global timer driver

- use in sabre (imx6) and zynq7000, the two platforms that were using
the private timer.