hw/arm/stellaris: Split stellaris-gptm into its own file

The implementation of the Stellaris general purpose timer module
device stellaris-gptm is currently in the same source file as the
board model.  Split it out into its own source file in hw/timer.

Apart from the new file comment headers and the Kconfig and
meson.build changes, this is just code movement.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Damien Hedde <damien.hedde@greensocs.com>
Message-id: 20210812093356.1946-24-peter.maydell@linaro.org