clocksource/drivers/tegra: Add verbose definition for 1MHz constant

Convert all 1MHz literals to a verbose constant for better readability.

Suggested-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Acked-by: Jon Hunter <jonathanh@nvidia.com>
Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Acked-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>