clocksource/drivers/tegra: Rename timer-tegra20.c to timer-tegra.c

Rename driver's source file to better reflect that it's not specific to
older SoC generations.

Suggested-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Acked-By: Peter De Schrijver <pdeschrijver@nvidia.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>