clockevents/drivers/tegra20: Remove obsolete inclusion of <asm/smp_twd.h>

As of commit da4a686a ("ARM: smp_twd: convert to use CLKSRC_OF
init"), this header file is no longer used.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>