Fix the below warning by making omap2_init_processor_devices() __init.
It is called by an __init function and calls only __init functions, so
it should also be init.

WARNING: arch/arm/mach-omap2/built-in.o(.text+0x183c): Section mismatch in reference from the function omap2_init_processor_devices() to the function .init.text:_init_omap_device()
The function omap2_init_processor_devices() references
the function __init _init_omap_device().
This is often because omap2_init_processor_devices lacks a __init
annotation or the annotation of _init_omap_device is wrong.
Reported-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Kevin Hilman <khilman@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>