soc: bcm: brcmstb: biuctrl: Change RAC prefetch distance from +/-1 to +/- 2

Change the RAC prefetch distance from +/- 1 to +/- 2 for Cortex-A72 CPUs
since this provides an average of a 3.8% performance increase for
synthetic memcpy benchmarks.

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