clocksource/integrator: Fix section mismatch warning

clocksource/integrator: Fix section mismatch warning

gcc-4.6 and older fail to inline integrator_clocksource_init, so they
end up showing a harmless warning:

WARNING: vmlinux.o(.text+0x4aa94c): Section mismatch in reference from the function integrator_clocksource_init() to the function .init.text:clocksource_mmio_init()
The function integrator_clocksource_init() references
the function __init clocksource_mmio_init().
This is often because integrator_clocksource_init lacks a __init
annotation or the annotation of clocksource_mmio_init is wrong.

Add the missing __init annotation that makes it build cleanly with all
compilers.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Thierry Reding <treding@nvidia.com>
Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: http://lkml.kernel.org/r/20170915194310.1170514-1-arnd@arndb.de