ARM: OMAP2+: mark omap_init_rng as __init

I found this section mismatch when building with an older
compiler release:

WARNING: vmlinux.o(.text+0x3051c): Section mismatch in reference from the
function omap_init_rng() to the function .init.text:omap_device_build()

Obviously this one function should be __init as well. Normally
we don't get a warning as the function gets inlined into its
caller.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
[tony@atomide.com: formatted error message a bit]
Signed-off-by: Tony Lindgren <tony@atomide.com>