ARM: bcm281xx: Move kona_l2_cache_init() so it can be shared

In preparation for future SoCs, move kona_l2_cache_init() from board
specific board_bcm281xx.c to shared kona.c, so multiple SoC families
can make use of it. Also change the return type to "void", since we
never look at the return code anyway.

Signed-off-by: Markus Mayer <markus.mayer@linaro.org>
Signed-off-by: Matt Porter <mporter@linaro.org>