ARM: bcm: rewrite commentary for bcm_kona_do_smc()

The block of comments in bcm_kona_do_smc() are somewhat confusing.
This patch attempts to clarify what's going on.

Signed-off-by: Alex Elder <elder@linaro.org>
Reviewed-by: Tim Kryger <tim.kryger@linaro.org>
Reviewed-by: Markus Mayer <markus.mayer@linaro.org>
Reviewed-by: Matt Porter <mporter@linaro.org>
Signed-off-by: Matt Porter <mporter@linaro.org>