soc: bcm: brcmstb: biuctrl: Enable Read-ahead cache

Brahma-B53 and Cortex-A72 CPUs integrated on Broadcom STB SoCs feature a
read-ahead cache that performs cache line size adaptation between the
bus interface unit and the memory controller.

On 32-bit ARM kernels we have to resort to a full featured read-ahead
cache driver under arch/arm/mm/cache-b15-rac.c (CONFIG_CACHE_B15_RAC)
because there are still cache maintenance operations by set/ways/index
that cannot be transparently handled by the ARM Coherency Extension that
the read-ahead cache interfaces to.

The 64-bit ARM kernel however has long deprecated all of those, so this
is simply a one time configuration.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>