ARM: 8729/1: Hook B15 readahead cache functions based on processor

If we detect that we are running on a Broadcom Brahma-B15 CPU, and
CONFIG_CACHE_B15_RAC is enabled, make sure that we pick-up the
b15_cache_fns function operations.

If CONFIG_CACHE_B15_RAC is enabled, but we are not running on a Broadcom
Brahma-B15 CPU, we will fallback to calling into the regular
v7_cache_fns with no cost. If CONFIG_CACHE_B15_RAC is disabled, there is
no cost and we just use the regular v7_cache_fns.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>