ARM: spear: add __init to spear_clocksource_init()

I get a build warning because spear_clocksource_init() calls
clocksource_mmio_init(), but it doesn't have an __init annotation.
Fix that.

Signed-off-by: Alex Elder <elder@linaro.org>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>