clocksource: Build Tegra timer on 32-bit ARM only

Instead of directly using the ARCH_TEGRA Kconfig symbol to enable this
driver, add a new, non-user-visible Kconfig symbol (TEGRA_TIMER) which
can be selected by the various SoCs.

This is useful to disable building the driver on Tegra132 (64-bit ARM)
where it doesn't currently compile but also isn't needed (yet).

Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>