clocksource: pxa: Fix section mismatch

As pxa_timer_common_init() is only called in init context, mark it as
such, and quiesce the compiler warnings :
WARNING: vmlinux.o(.text.unlikely+0x45d4): Section mismatch in reference
from the function pxa_timer_common_init() to the function
.init.text:sched_clock_register()

WARNING: vmlinux.o(.text.unlikely+0x4610): Section mismatch in reference
from the function pxa_timer_common_init() to the function
.init.text:clocksource_mmio_init()

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>