clocksource/timer_of: Rename timer_of_exit to timer_of_cleanup

Change the function name to something more explicit since it is only used
in init error cases.

Add __init annotation and description about the function usage.

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: mark.rutland@arm.com
Cc: devicetree@vger.kernel.org
Cc: alexandre.torgue@st.com
Cc: arnd@arndb.de
Cc: julien.thierry@arm.com
Cc: daniel.lezcano@linaro.org
Cc: linux@armlinux.org.uk
Cc: robh+dt@kernel.org
Cc: linux-arm-kernel@lists.infradead.org
Cc: mcoquelin.stm32@gmail.com
Cc: sudeep.holla@arm.com
Cc: ludovic.barre@st.com
Link: https://lkml.kernel.org/r/1510649563-22975-2-git-send-email-benjamin.gaignard@linaro.org