clocksource/drivers: Unify the names to timer-* format

In order to make some housekeeping in the directory, this patch renames
drivers to the timer-* format in order to unify their names.

There is no functional changes.

Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Vladimir Zapolskiy <vz@mleia.com>
Acked-by: Liviu Dudau <liviu.dudau@arm.com>

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>