soc: bcm: brcmstb: biuctrl: Change RAC data line prefetching after 4 consecutive lines

Change the RACPREFDATA(x) setting to prefetch the next 256-byte line
after 4 consecutive lines have been used, instead of after 2 consecutive
lines. This does improve the synthetic memcpy benchmark by an additional
+0.5% on top of the previous change for Cortex-A72 CPUs.

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