ARM: bcm: fix missing include of kona_l2_cache.h

Fix a warning of kona_l2_cache_init() not being declared by
including the header file kona_l2_cache.h which defines it
thus fixing:

arch/arm/mach-bcm/kona_l2_cache.c:21:13: warning: symbol 'kona_l2_cache_init' was not declared. Should it be static?

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Acked-by: Ray Jui <ray.jui@broadcom.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>