clocksource/drivers/tegra186: Put Kconfig option 'tristate' to 'bool'

The clocksources are built-in, they are not modules. We don't know if
the core time framework is ready for clockevents / clocksources as
modules.

Revert back this option to 'bool'.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20220718213657.1303538-1-daniel.lezcano@linaro.org