ARM: tegra: build localtimer support only when needed

It is possible to build a tegra kernel without localtimer
support, so the tegra specific parts should only be built
when that is indeed enabled.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Alan Ott <alan@signal11.us>
Signed-off-by: Olof Johansson <olof@lixom.net>